docs: Enable strict mode

The strict mode fails if `mkdocs` produces warnings.
This helps to identify broken links or other errors.

.github/workflows/docs.yml

@@ -1,18 +1,18 @@
 # SPDX-FileCopyrightText: Copyright DB InfraGO AG and contributors
 # SPDX-License-Identifier: CC0-1.0
 
-name: Development documentation
+name: documentation
 on:
   push:
-    branches: [ main ]
+    branches: [main]
 jobs:
   deploy:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v2
         with:
-          python-version: "3.11"
+          python-version: '3.11'
       - run: pip install mkdocs-material
-      - run: mkdocs gh-deploy --force
+      - run: mkdocs gh-deploy --strict --force
         working-directory: ./docs

docs/Makefile

@@ -7,3 +7,6 @@ install:
 
 serve:
 	.venv/bin/mkdocs serve
+
+build:
+	.venv/bin/mkdocs build --strict