Update CI workflow to deploy to GitHub Pages.

Since the current colcon maintainers do not have administrative access
to colcon.readthedocs.org this is an additional deployment configuration
that can target eventually target colcon.github.io

* Updates the CI build to run on pull requests.
* Adds upload and deploy jobs which use the build results.

.github/workflows/ci.yml

@@ -1,6 +1,9 @@
 name: colcon.readthedocs.org
 
-on: [push]
+on:
+  push:
+    branches: ['main']
+  pull_request:
 
 jobs:
   build:
@@ -36,3 +39,29 @@ jobs:
         if [[ -s "_build/spelling/index.spelling" ]]; then cat "_build/spelling/index.spelling"; fi
         if [[ -s "_build/warnings_and_errors.log" ]]; then false; fi
         if [[ -s "_build/spelling/index.spelling" ]]; then false; fi
+
+    upload:
+      if: ${{ github.event_name == "push" }}
+      needs: build
+      runs-on: ubuntu-22.04
+      steps:
+        - name: Upload build for GitHub Pages
+          uses: actions/upload-pages-artifact@v3
+          with:
+            path: _build/html
+
+    deploy:
+      permissions:
+        contents: read
+        id-token: write
+        pages: write
+      environment:
+        name: github-pages
+        url: ${{ steps.deployment.outputs.page_url }}
+      if: ${{ github.event_name == "push" }}
+      needs: upload
+      runs-on: ubuntu-22.04
+      steps:
+        - name: Deploy to GitHub Pages
+          id: deployment
+          uses: actions/deploy-pages@v4