docs(nav): publish every ADR and runbook, gated by a nav check (closes #169) (#172)

## What & why

`docs/` is the source of truth (CLAUDE.md §12), but only pages listed in `mkdocs.yml`'s nav
are published — and mkdocs' own `validation.nav.omitted_files: warn` keeps the build green
while dropping the rest. So the site had quietly stopped at **ADR-0010** and
**`runbooks/ci.md`**: 31 pages, including every ADR from 0011 to 0034, six of the seven
runbooks, and `synthetic-data.md`, existed in the repo and nowhere else.

- `infra/check-docs-nav.py` fails when a page under `docs/` is not in the nav. It runs in
  `make lint`, so the existing CI job gates it — python3 only, no new tooling, and no
  mkdocs install needed to check it.
- The nav now lists all 34 ADRs, all 7 runbooks and `synthetic-data.md`.
- ADR-0033's `Slice:` header said "none yet"; #25 closed it.
- The landing page gained a pointer to the Talos runbook.

Closes #169

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the fix — the red commit lists all 31 missing pages.
- [x] Implementation makes the test pass.
- [x] Conventional Commits referencing the issue (`refs #169`).
- [ ] CI green — awaiting the run on this PR (`python3 infra/check-docs-nav.py` passes locally;
      `make lint` also needs the .NET SDK, which CI has).
- [x] `docker compose up` unaffected — docs and `mkdocs.yml` only, plus one `make lint` line.
- [x] Docs updated — that is the change.
- [x] No ADR needed: no dependency, no boundary, no §8 rule touched.
- [x] Not user-visible, so no demo note.

## Notes for reviewers

- The check is a **substring test**, not a YAML parse (there's a `ponytail:` note in the
  script): a page's path either appears in `mkdocs.yml` or it doesn't. That keeps it
  dependency-free — `mkdocs.yml` can't be read by `yaml.safe_load` anyway, it carries a
  `!!python/name:` tag for the mermaid fence. It does not check that an entry *points at a
  file that exists*; mkdocs' `not_found: warn` covers that direction.
- ADR labels in the nav are shortened by hand (`"ADR-0013: Behandel-portal wiring"`), since
  several H1s are a full sentence.

**Known gap, not fixed here:** CLAUDE.md §12 says the site is "published via a Gitea Actions
workflow to Gitea Pages", and no such workflow exists — `mkdocs build` is never run, by CI or
by any make target. Gitea has no built-in Pages, so publishing needs a decision (a
`gitea-pages` server, an artifact, or a static host) rather than a patch. Worth its own issue
if the published site is actually wanted; until then this PR makes the nav correct for whoever
runs `mkdocs serve`.Reviewed-on: #172

Makefile

@@ -64,6 +64,9 @@ frontend:
 ## lint: verify formatting (no changes)
 lint:
 	dotnet format $(SLN) --verify-no-changes
+	# Only pages in mkdocs.yml's nav are published, and mkdocs keeps a build green
+	# when one is missing — so the nav is checked here rather than not at all.
+	python3 infra/check-docs-nav.py
 
 ## build: release build
 build: