ci: Add GitHub Actions workflow for deploying docs to Pages

Builds the mdbook documentation and deploys to GitHub Pages on
pushes to main that modify book/ files. Also supports manual
dispatch via workflow_dispatch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docs.yml

@@ -0,0 +1,54 @@
+name: Deploy Documentation
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - 'book/**'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    name: Build mdbook
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install mdbook
+        uses: taiki-e/install-action@v2
+        with:
+          tool: mdbook
+
+      - name: Build book
+        run: mdbook build book
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: book/build
+
+  deploy:
+    name: Deploy to GitHub Pages
+    needs: build
+    runs-on: ubuntu-latest
+
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4