Switch to deploying via GitHub Actions

This will enable us to move to Ruby 3 as well as offering other
freedoms to customise our build process.

This config is copied from the docs at srobo/docs#588.

.github/workflows/publish.yml

@@ -0,0 +1,70 @@
+name: Build & Publish
+
+on:
+  push:
+    branches:
+      - main
+  pull_request: {}
+
+jobs:
+  build-validate:
+    env:
+      # ruby/setup-ruby@v1 does not offer a way to change the cached gems path.
+      # See https://github.com/ruby/setup-ruby/issues/291
+      GLOBAL_GEMS: 1
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+          cache-version: 2
+
+      - name: Build & Validate
+        run: rake validate
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Build container
+      run: docker-compose build
+
+    - name: Build site in container
+      run: docker-compose run docs rake build
+
+  deploy:
+    name: Deploy to GitHub Pages
+    runs-on: ubuntu-latest
+    needs: build-validate
+    if: ${{ github.ref == 'refs/heads/main' }}
+
+    # Allow only one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
+    # However, do NOT cancel in-progress runs as we want to allow these production deployments to complete.
+    concurrency:
+      group: "pages"
+      cancel-in-progress: false
+
+    # Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
+
+    environment:
+      name: github-pages
+      # Tell GitHub where we deployed to
+      url: ${{ steps.deployment.outputs.page_url }}
+
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

.ruby-version

@@ -1 +1 @@
-2.5.1
+2.7.3