automation: add workflow to fail on warnings during docs build

.github/workflows/test-docs.yaml

@@ -0,0 +1,30 @@
+# This is a GitHub workflow defining a set of jobs with a set of steps.
+# ref: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions
+#
+name: Test docs
+
+on:
+  pull_request:
+  push:
+    branches-ignore:
+      - "dependabot/**"
+      - "pre-commit-ci-update-config"
+  workflow_dispatch:
+
+jobs:
+  test-docs:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install deps
+        run: pip install -r requirements.txt
+
+      # linkcheck is done separately from this in doc-links.yml, scheduled to
+      # run every day and open/update an issue
+      - name: make dirhtml
+        run: |
+          make dirhtml SPHINXOPTS='--color -W --keep-going'