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 website, with the minor adjustment
that it uses the latest versions of the pages actions.

Author: PeterJCLaw

.github/workflows/publish.yml

@@ -0,0 +1,70 @@
+name: Build & Publish
+
+on:
+  push:
+    branches:
+      - main
+  pull_request: {}
+
+jobs:
+  build:
+    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
+    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

.github/workflows/validate.yml

@@ -0,0 +1 @@
+2.7.3