test(docs): fail when a page under docs/ is missing from the nav (refs #169)

Only the pages listed in mkdocs.yml's nav are published, and mkdocs' own
`omitted_files: warn` keeps the build green while dropping the rest — so nothing
ever said that the site had quietly stopped at ADR-0010 and runbooks/ci.md.

Runs in `make lint` (python3 only, no new tooling), so it gates every PR through
the existing job.

Red: 31 pages are missing — ADR-0011 through ADR-0034, six of the seven runbooks,
and synthetic-data.md.

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: