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

Red: the strict build currently fails on a link out of docs_dir in
runbooks/ci.md.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.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