ci: add mkdocs build --strict step

The docs have just been cleaned so the MkDocs site builds strictly
with zero warnings. Wire it into the lint-and-test matrix so any
future link rot, broken ``:::`` reference, or missing nav entry
aborts CI instead of silently degrading the hosted docs.

Author: Yuma-Ichikawa

.github/workflows/ci.yml

@@ -37,3 +37,6 @@ jobs:
 
       - name: Run tests
         run: uv run pytest -q
+
+      - name: MkDocs strict build
+        run: uv run mkdocs build --clean --strict