(docs) Do not push docs on non-main branch

Still generates docs because this is (or, will be) an alternative way to
test stuff.

Vibe coded so don't ask. TBH I don't know much about Github Workflows so
there will be tinkering, vibe coded or not.

Author: ahyangyi

.github/workflows/sphinx-gh-pages.yml

@@ -45,6 +45,7 @@ jobs:
         path: './docs/_build/html'
 
   deploy:
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}