Publish site via GitHub Actions

GitHub is shutting down the legacy build system towards the end of
June. Migrate to using GitHub Actions.

Add a test workflow for pull requests alongside it.

This also gives us access to:
- Updated Jekyll
- Updated Ruby
- Updated literally everything because GitHub's built-in thing was so
  old everything was crying in pain

This also *probably* makes it build faster, but untested on GitHub...

.github/workflows/publish.yml

@@ -0,0 +1,36 @@
+name: Deploy site
+
+on:
+  push:
+    branches: [ master ]
+  workflow_dispatch:
+
+jobs:
+  doc-deploy:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.2'
+          bundler-cache: true
+
+      - name: Run Jekyll
+        # Build the site using Jekyll
+        run: JEKYLL_ENV=production bundle exec jekyll build
+
+        # Copy (or touch) other needed files
+        # .nojekyll: to tell GH Pages to not rebuild (lol)
+      - name: Post-Jekyll build copy files
+        run: touch _site/.nojekyll
+
+      - name: Deploy 🚀
+        uses: JamesIves/github-pages-deploy-action@v4
+        with:
+          branch: gh-pages # The branch the action should deploy to.
+          folder: _site # The folder the action should deploy.
+          clean: true # Automatically remove deleted files from the deploy branch

.github/workflows/test.yml

@@ -0,0 +1,24 @@
+name: Test site build
+
+on:
+  pull_request:
+
+jobs:
+  doc-test:
+    if: github.ref_name != 'l10n'   
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+          submodules: recursive
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.2'
+          bundler-cache: true
+
+      - name: Run Jekyll
+        # Build the site using Jekyll
+        run: JEKYLL_ENV=production bundle exec jekyll build