add workflow to test docs build on pr

.github/workflows/build-docs.yml

@@ -0,0 +1,26 @@
+# Build the Sphinx docs on PR to catch any issues before merging.
+# Deployment happens on push to main with gh-pages.yml
+name: Build Docs
+
+on:
+  pull_request:
+    branches: [main]
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+jobs:
+  pages:
+    runs-on: ubuntu-20.04
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    permissions:
+      pages: write
+      id-token: write
+    steps:
+    - id: deployment
+      uses: sphinx-notes/pages@v3
+      with:
+        publish: false