docs: add a Customize your template series under Get started (#26712)

## What

Adds a **Customize your template** series under the top-level **Get
started** section, at `docs/get-started/customize-your-template/`.
These guides extend the single-page Quickstart (added in #26821) with
hands-on template customization:

- **Add a programming language** — expose a language through a
parameter, install it at startup, and offer it as a preset.
- **Install your own command-line tools** — install personal tools with
Homebrew and mise, and make them persist.
- **Clone private repositories** — authenticate workspaces to GitHub
with an external-auth data source.

## Changes from the earlier draft

This branch was rebased onto the consolidated `/docs/get-started`
structure:

- Re-homed the series from `tutorials/quickstart/` to
`get-started/customize-your-template/`, nested under the new Get started
section.
- Dropped the Part 1 launch page and the old landing page; the merged
Quickstart (`get-started/index.md`, #26821) already covers them.
- Archived the dotfiles guide out of the series (tracked as a follow-up
to document the dotfiles module as a standalone tutorial) and removed
its inbound links.
- Renamed the section to **Customize your template**.
- Added a Ruby **preset** alongside the Ruby parameter so the parameter
and preset choices stay in sync.
- Fixed the parameter-change steps to route through **Workspace settings
> Parameters**.
- Gave each page a **What's next?** step so the series reads as a
sequence.

## Still open / follow-ups

- Screenshots for the UI steps (handled separately).
- The launch step will be revised after the template-builder change
ships in the next mainline release.

<details>
<summary>Decision log</summary>

- **Why re-home, not keep `tutorials/quickstart/`:** the Quickstart now
lives at `/docs/get-started`, so the series belongs under the same
top-level section for a single, coherent entry point.
- **Why drop Part 1 here:** the launch content already merged as
`get-started/index.md` in #26821; keeping a second copy would duplicate
and drift.
- **Why archive dotfiles:** it works better as a standalone module
tutorial than as a Quickstart step; removed from the series for now and
tracked for later.
- **Final code fixtures** stay scoped per guide (base template plus that
page's edit), so only the language guide's fixture gains the Ruby option
and preset.

</details>

---
Generated by Coder Agents on behalf of @nickvigilante.

docs/manifest.json

@@ -86,7 +86,31 @@
 			"title": "Get started",
 			"description": "Install Coder and launch your first workspace",
 			"path": "./get-started/index.md",
-			"icon_path": "./images/icons/rocket.svg"
+			"icon_path": "./images/icons/rocket.svg",
+			"children": [
+				{
+					"title": "Customize your template",
+					"description": "Add a language, install command-line tools, and authenticate to GitHub",
+					"path": "./get-started/customize-your-template/index.md",
+					"children": [
+						{
+							"title": "Add a programming language",
+							"description": "Expose a new language through a parameter and install it at startup",
+							"path": "./get-started/customize-your-template/add-a-language.md"
+						},
+						{
+							"title": "Install your own command-line tools",
+							"description": "Install personal command-line tools and make them persist",
+							"path": "./get-started/customize-your-template/install-command-line-tools.md"
+						},
+						{
+							"title": "Clone private repositories",
+							"description": "Authenticate to GitHub with an external-auth data source",
+							"path": "./get-started/customize-your-template/authenticate-to-github.md"
+						}
+					]
+				}
+			]
 		},
 		{
 			"title": "Install",