Add preview site for pull requests

Pin Ruby version.

Assisted-by: GitHub Copilot
Assisted-by: OpenAI GPT-5.1-Codex

Author: Sjors

.github/workflows/preview.yml

@@ -0,0 +1,48 @@
+name: Preview Site
+
+on:
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages-preview-${{ github.ref }}"
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out source
+        uses: actions/checkout@v4
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+      - name: Build site
+        run: bundle exec jekyll build --trace --destination _site
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: _site
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy preview
+        id: deployment
+        uses: actions/deploy-pages@v4
+        with:
+          preview: true

.ruby-version

@@ -0,0 +1 @@
+3.1.2