ci(docs): build the MkDocs site with --strict in CI (refs #173) (#189)

refs #173. This is the minimum step from the issue: the site is now **built** in CI, not **published**. Publishing still needs an ADR (Gitea has no built-in Pages) or a CLAUDE.md §12 correction, so the issue stays open.

- `make docs`: creates a throwaway `.venv-docs`, installs pinned `mkdocs==1.6.1` and `mkdocs-material==9.7.7`, then runs `mkdocs build --strict`. The target is also added to `make ci`.
- New `docs` job in `ci.yaml` (`setup-python@v5`, then `make docs`).
- Red, then green: the first commit fails on a link from `runbooks/ci.md` to a file outside `docs/`; the second turns that link into plain code.

**Dependency (§13):** mkdocs and mkdocs-material were already the site's declared toolchain (`mkdocs.yml`) but were never installed anywhere. They give a strict link/nav/theme check. Replacing them means writing our own Markdown link checker, and `check-docs-nav.py` already covers only the nav half. Risk: Material warns that MkDocs 2.0 drops its plugin/theme system, so both are pinned exactly. No ADR, since this adds no new decision beyond what `mkdocs.yml` already assumes.

Verified locally: `make docs` → `Documentation built in 0.87 seconds`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #189

.gitea/workflows/ci.yaml

@@ -98,6 +98,17 @@ jobs:
           [ -n "${GITHUB_STEP_SUMMARY:-}" ] || exit 0
           python3 infra/trx-summary.py TestResults >> "$GITHUB_STEP_SUMMARY"
 
+  # The docs site must build with --strict (#173). setup-python so `make docs` can
+  # create its venv regardless of what the runner image ships.
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: https://github.com/actions/checkout@v4
+      - uses: https://github.com/actions/setup-python@v5
+        with:
+          python-version: '3.12'
+      - run: make docs
+
   # Frontend (Nx/Angular) lane: install with pnpm, then Nx lint + test + build.
   frontend:
     runs-on: ubuntu-latest