refactor(docs): convert absolute coder/coder blob/tree/main links to relative (DOCS-351) (#26341)

Closes [DOCS-351](https://linear.app/codercom/issue/DOCS-351).

> [!WARNING]
> **DO NOT MERGE** until
[DOCS-349](https://linear.app/codercom/issue/DOCS-349)
([coder.com#877](https://github.com/coder/coder.com/pull/877)) has
shipped to production and baked for at least one Vercel cycle.
>
> Without DOCS-349, the relative links in this PR resolve to broken
docs-route URLs (`/docs/helm/coder/values.yaml` -> 404) instead of
GitHub URLs tagged with the displayed docs version. DOCS-349 fixes the
rewriter to classify these as GitHub blob/tree URLs with the page's
resolved ref.

## TL;DR

Converts 121 absolute
`https://github.com/coder/coder/(blob|tree)/main/<path>` links across 39
docs markdown files to relative paths. After this lands AND DOCS-349
deploys, every one of these links will follow the displayed docs version
(mainline tag on bare URLs, explicit tag on `/@vX.Y.Z/`, `main` on
`/@main/`) instead of always pointing to `main`.

## Why

Today a reader on `/docs/@v2.30.0/install/docker` follows a
`compose.yaml` link and arrives at `main`'s `compose.yaml`, which
doesn't necessarily match what the docs page describes. Helm values,
Terraform templates, and source-code references in particular drift
across versions. The fix is to let the coder.com rewriter substitute the
page's resolved ref into the URL; that only works on relative links.

## Example payoff (post-DOCS-349)

| URL | Today (absolute, always `main`) | After (relative + rewriter) |
|---|---|---|
| `/docs/install/docker` |
`https://github.com/coder/coder/blob/main/compose.yaml` |
`https://github.com/coder/coder/blob/v2.34.1/compose.yaml` (today's
mainline) |
| `/docs/@v2.30.0/install/docker` | same as above |
`https://github.com/coder/coder/blob/v2.30.0/compose.yaml` |
| `/docs/@main/install/docker` | same as above |
`https://github.com/coder/coder/blob/main/compose.yaml` |

## Scope

- **121 conversions** across **39 files**.
- Verb breakdown: `tree/main` (directories) and `blob/main` (files),
both flipped to relative paths.
- Line anchors (`#L23-L24`) and query strings preserved verbatim.
- Conversion is mechanical: relative path computed from the doc file's
directory to the target via `os.path.relpath`. Any path starting at the
same directory or below gets a `./` prefix; otherwise `../` chains.

## Rebased on main

The branch was rebased onto `main` after the DOCS-350 hotfix
([#26339](https://github.com/coder/coder/pull/26339)) merged. The hotfix
repointed 3 `docs-backend-contrib-guide` refs in `backend.md` to `main`,
which then needed the same `main` -> relative conversion this PR is
doing for the other 121 links. The conflict was resolved by reapplying
the mechanical conversion to `backend.md` after taking the hotfix's
content. Net result: those 3 links land here as relative, same as
everything else. New HEAD `3f501cb622`.

## Inline fix folded in: dead `nix` link

- `docs/about/contributing/CONTRIBUTING.md:7` -> `../../../nix`

The original absolute URL `https://github.com/coder/coder/tree/main/nix`
already returned 404 today. Repointed to `flake.nix` (modern Nix
entrypoint, what the prose "Nix environment" semantically refers to).
Closes [DOCS-357](https://linear.app/codercom/issue/DOCS-357) here since
the `check-docs` Linkspector job surfaced it during rebase; cheaper to
fix inline than in a separate single-line PR.

## Out of scope (filed separately)

- [DOCS-350](https://linear.app/codercom/issue/DOCS-350): 3 dead
`docs-backend-contrib-guide` branch refs in `backend.md`
([#26339](https://github.com/coder/coder/pull/26339), merged).
- [DOCS-352](https://linear.app/codercom/issue/DOCS-352): 10 SHA-pinned
`(blob|tree)/<sha>` links pending intent review.
- [DOCS-355](https://linear.app/codercom/issue/DOCS-355): code-server
analog (4 absolute `(blob|tree)/main` links in `coder/code-server`).
- [DOCS-356](https://linear.app/codercom/issue/DOCS-356): 2 upstream
content bugs in `coder/code-server/docs/CONTRIBUTING.md` (independent of
this PR).


## Not triggering `/coder-agents-review`

Docs-only edit; per `AGENTS.md` the bot review is reserved for
product/CI changes.

## Pre-mortem

| Concern | Mitigation |
|---|---|
| Merging before DOCS-349 deploys regresses ~120 currently-working links
into 404s on coder.com | Clear DO-NOT-MERGE banner; tracked as blocker
in Linear. |
| Relative path computed incorrectly (off-by-one `..`) | Verified all
114 newly-relative non-md/non-image paths resolve to existing files in
the repo (only exception is the pre-existing dead `nix` link above). |
| Line anchors stripped during conversion | Preserved by the
substitution regex; verified `#L<n>-L<m>` cases in `airgap.md` and
`speed-up-templates.md`. |
| Future code reorgs change file locations | Relative links will start
pointing to nothing. Same failure mode as absolute links pointing to
renamed files; can be caught with a future link-checker job. |

## Validation

```
$ grep -rE 'github\.com/coder/coder/(blob|tree)/main' docs --include="*.md" | wc -l
0
$ git diff --stat origin/main | tail -1
39 files changed, 118 insertions(+), 118 deletions(-)
```

114 newly-relative paths verified to resolve to existing repo files
(Python `os.path.exists` check on each computed target).

<details>
<summary>Decision log + planning context</summary>

**Why relative over `(blob|tree)/{{currentDocsVersion}}/...`
templating**: relative paths require zero markdown-system support and
zero upstream churn beyond this one PR. Templating would require a
preprocessor on `coder.com` side AND a convention upstream authors have
to remember; relative paths just work in a plain editor and
`github.com`'s own renderer too.

**Why `./` prefix on same-directory targets**: makes the conversion
grep-able later (`grep -E '\((\.\./|\./)'`).

**Why preserve `#L<n>-L<m>` anchors verbatim**: the anchor is meaningful
to the linked file's content, not to the URL form; keeping it as-is
preserves authorial intent. If the file later changes such that the line
range drifts, that's a different problem the SHA-pin audit
([DOCS-352](https://linear.app/codercom/issue/DOCS-352)) will surface.

</details>

---

*Generated by Coder Agents on @nickvigilante's behalf.*





## Drive-by external link fix folded in

`docs/about/contributing/CONTRIBUTING.md:296` cited
`https://reflectoring.io/meaningful-commit-messages/` which is returning
HTTP 503 (the host appears to be down site-wide right now). `check-docs`
Linkspector flagged it after the rebase. Replaced with
`https://cbea.ms/git-commit/` (Chris Beams' canonical "If applied, this
commit will..." article, confirmed 200), which is the original source of
the rule the prose recites anyway.

docs/admin/infrastructure/scale-utility.md

@@ -4,7 +4,7 @@ We scale-test Coder with a built-in utility that can
 be used in your environment for insights into how Coder scales with your
 infrastructure. For scale-testing Kubernetes clusters we recommend that you install
 and use the dedicated Coder template,
-[scaletest-runner](https://github.com/coder/coder/tree/main/scaletest/templates/scaletest-runner).
+[scaletest-runner](../../../scaletest/templates/scaletest-runner).
 
 Learn more about [Coder’s architecture](./architecture.md) and our
 [scale-testing methodology](./scale-testing.md).
@@ -138,7 +138,7 @@ This will delete all workspaces and users with the prefix `scaletest-`.
 ## Scale testing template
 
 Consider using a dedicated
-[scaletest-runner](https://github.com/coder/coder/tree/main/scaletest/templates/scaletest-runner)
+[scaletest-runner](../../../scaletest/templates/scaletest-runner)
 template alongside the CLI utility for testing large-scale Kubernetes clusters.
 
 The template deploys a main workspace with scripts used to orchestrate Coder,
@@ -177,7 +177,7 @@ Scale testing concurrency can be controlled with the following parameters:
 It is recommended to learn how to operate the _scaletest-runner_ before running
 it against the staging cluster (or production at your own risk). Coder provides
 different
-[workspace configurations](https://github.com/coder/coder/tree/main/scaletest/templates)
+[workspace configurations](../../../scaletest/templates)
 that operators can deploy depending on the traffic projections.
 
 There are a few cluster options available:
@@ -205,7 +205,7 @@ Use this template variant to verify limits of the cluster performance.
 
 During scale tests, operators can monitor progress using a Grafana dashboard.
 Coder offers a comprehensive overview
-[dashboard](https://github.com/coder/coder/blob/main/scaletest/scaletest_dashboard.json)
+[dashboard](../../../scaletest/scaletest_dashboard.json)
 that can seamlessly integrate into the internal Grafana deployment.
 
 This dashboard provides insights into various aspects, including:

docs/admin/infrastructure/validated-architectures/index.md

@@ -157,7 +157,7 @@ to schedule the control plane pods on the appropriate node group.
 
 Coder workspaces can be deployed either as Pods or Deployments in Kubernetes.
 See our
-[example Kubernetes workspace template](https://github.com/coder/coder/tree/main/examples/templates/kubernetes).
+[example Kubernetes workspace template](../../../../examples/templates/kubernetes).
 Configure the workspace node group to be auto-scaling, to dynamically allocate
 compute as users start/stop workspaces at the beginning and end of their day.
 Set nodeSelectors, affinities, and tolerations in Coder templates to assign
@@ -378,7 +378,7 @@ could affect workspace users experience once the platform is live.
 ### Helm Chart Configuration
 
 1. Reference our
-   [Helm chart values file](https://github.com/coder/coder/blob/main/helm/coder/values.yaml)
+   [Helm chart values file](../../../../helm/coder/values.yaml)
    and identify the required values for deployment.
 1. Create a `values.yaml` and add it to your version control system.
 1. Determine the necessary environment variables. Here is the