Build documentation in pull requests

This will catch basic issues in PRs.

.github/workflows/deploy-docs.yml

@@ -1,11 +1,12 @@
-name: Deploy
+name: Documentation
 on:
   push:
     branches: [main]
+  pull_request:
 
 jobs:
   docs:
-    name: Documentation
+    name: Build
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
@@ -18,6 +19,7 @@ jobs:
         run: nox -s docs
 
       - name: Upload to GitHub Pages
+        if: github.ref == 'refs/heads/main' && github.repository == 'pradyunsg/furo'
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}