(MAINT) Build docs on PR, Publish on push

This change modifies the GHA for the docs site to build the
docs on a PR but only publish them on a push to `main`.

This way, future docs changes that break the build can be
caught before a merge.

.github/workflows/hugo.yaml

@@ -6,6 +6,7 @@ on:
   push:
     branches:
       - main
+  pull_request:
 
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch:
@@ -78,6 +79,7 @@ jobs:
 
   # Deployment job
   deploy:
+    if: github.ref == 'refs/heads/main'
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}