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

Makefile

@@ -43,11 +43,11 @@ export DOCKER_HOST := unix://$(PODMAN_SOCK)
 endif
 endif
 
-.PHONY: ci lint build unit mutation frontend integration verify verify-up verify-acl verify-nrc verify-projection verify-bff verify-domain verify-observability verify-tracing verify-metrics verify-objecttypen verify-objecten verify-registerrecord verify-objecten-notifications verify-notifications smoke up down local verify-local local-down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down flowable-up flowable-smoke flowable-down k8s-lint k8s-drift k8s-registry k8s-images k8s-seed k8s-up k8s-reseed k8s-portals k8s-down k8s-purge help
+.PHONY: ci lint build unit mutation frontend docs integration verify verify-up verify-acl verify-nrc verify-projection verify-bff verify-domain verify-observability verify-tracing verify-metrics verify-objecttypen verify-objecten verify-registerrecord verify-objecten-notifications verify-notifications smoke up down local verify-local local-down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down flowable-up flowable-smoke flowable-down k8s-lint k8s-drift k8s-registry k8s-images k8s-seed k8s-up k8s-reseed k8s-portals k8s-down k8s-purge help
 
 ## ci: run the full pipeline — lint, build, unit, mutation, frontend, verify (mirrors Gitea Actions)
 ## `verify` is the live-stack stage (full stack up once → ACL + notification checks).
-ci: lint build unit mutation frontend verify
+ci: lint build unit mutation frontend docs verify
 
 ## frontend: install deps and run the Nx lint/test/build for the portals (pnpm + Node required)
 # Tests run in their own phase, ahead of the build. The @angular/build:unit-test
@@ -81,6 +81,15 @@ unit:
 	python3 infra/test_playwright_summary.py
 	python3 infra/test_portal_caddyfiles.py
 
+## docs: build the MkDocs site with --strict (a broken link or nav entry fails)
+# Pinned in a throwaway venv: Material 9.7 is the last line on MkDocs 1.x, and MkDocs
+# 2.0 drops the plugin/theme system this site relies on. Publishing is a separate
+# decision (#173); this only proves the site builds.
+docs:
+	python3 -m venv .venv-docs
+	.venv-docs/bin/pip install --quiet mkdocs==1.6.1 mkdocs-material==9.7.7
+	.venv-docs/bin/mkdocs build --strict
+
 ## mutation: run the Stryker.NET ratchet on each service with branching logic (fails below baseline)
 # Stryker is pinned as a local dotnet tool (.config/dotnet-tools.json); `tool restore`
 # makes `make mutation` work from a fresh clone. Each service owns its config + break