ci: modify docs job to run on pull requests but only push on main

This should ensure that docs will still build on any merge but only push the generated HTML once it hits main.

.github/workflows/docs.yaml

@@ -4,6 +4,9 @@ on:
       - main
     tags-ignore:
       - "**-rc**"
+  pull_request:
+    branches:
+      - main
 
 name: Deploy DataFusion Python site
 
@@ -13,6 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Set target branch
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         id: target-branch
         run: |
           set -x
@@ -27,6 +31,7 @@ jobs:
       - name: Checkout docs sources
         uses: actions/checkout@v3
       - name: Checkout docs target branch
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         uses: actions/checkout@v3
         with:
           fetch-depth: 0
@@ -64,6 +69,7 @@ jobs:
           make html
 
       - name: Copy & push the generated HTML
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         run: |
           set -x
           cd docs-target