CI: Build Sphinx docs in nitpicky mode

This would've caught an accidentally commited markdown file.

.github/workflows/on-push.yml

@@ -3,6 +3,19 @@ name: on-push
 on: [push]
 
 jobs:
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Set up Python
+      uses: actions/setup-python@v3
+      with:
+        python-version: 3.12
+    - name: Install dependencies
+      run: |
+        pip install .[docs]
+    - name: Build Sphinx documentation
+      run: sphinx-build -n -W --keep-going doc doc/_build
   static_checks:
     runs-on: ubuntu-latest
     steps: