docs: fix broken links in weekly-docs link check (#26813)

Fix four broken links that caused the weekly-docs link-check CI job to
fail.

**Changes:**

- `docs/install/rancher.md`: Remove `#readme` anchor from
`../../helm#readme` — linkspector splits on `#`, finds a directory, and
errors with EISDIR.
- `docs/install/kubernetes.md`: Same fix for `../../helm/coder#readme`.
- `docs/install/cloud/compute-engine.md`: Point both `gcp-linux` links
to `README.md` explicitly
(`../../../examples/templates/gcp-linux/README.md` and
`../../../examples/templates/gcp-linux/README.md#authentication`) so
linkspector can resolve the file and anchor.
- `.github/.linkspector.yml`: Add `merriam-webster.com` to
`ignorePatterns` (returns 403 from GitHub runner IPs).

<details>
<summary>Linear issue and CI context</summary>

**Linear issue:**
https://linear.app/codercom/issue/DOCS-494/fix-broken-links-in-weekly-docs-link-check

**Failing CI run:**
https://github.com/coder/coder/actions/runs/28366176335/job/84032582533

The workflow is `.github/workflows/weekly-docs.yaml`, job `check-docs`,
step `Check Markdown links` (umbrelladocs/action-linkspector).

Root causes confirmed per investigation:
- `#readme` anchors on directory paths trigger EISDIR in linkspector's
local resolver.
- The `gcp-linux` directory links needed explicit `README.md` targets;
linkspector cannot resolve bare directory references.
- `merriam-webster.com` blocks GitHub runner IPs with 403.

`ignorePatterns` is reserved for external links only, not internal or
GitHub file links.

</details>

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

.github/.linkspector.yml

@@ -34,5 +34,7 @@ ignorePatterns:
   - pattern: "platform.openai.com"
   - pattern: "api.openai.com"
   - pattern: "openai.com"
+  # merriam-webster.com returns 403 from GitHub runner IPs
+  - pattern: "merriam-webster.com"
 aliveStatusCodes:
   - 200