Add GitHub Pages workflow to deploy docs

Introduce .github/workflows/deploy-docs.yml to build the site with Astro and deploy to GitHub Pages. The workflow runs on push to the temporary branch `gh-pages-action` (TODO: switch back to `main` after verification) and supports manual dispatch. It grants minimal permissions for pages deployment, enforces concurrency to avoid overlapping runs, builds the `docs` site using withastro/action, and deploys via actions/deploy-pages@v5.

Author: JR-1991

.github/workflows/deploy-docs.yml

@@ -0,0 +1,38 @@
+name: Deploy docs to GitHub Pages
+
+on:
+  push:
+    branches: [gh-pages-action]   # TODO: switch back to [main] once verified
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Allow one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+      - name: Build with Astro
+        uses: withastro/action@v6
+        with:
+          path: docs
+
+  deploy:
+    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@v5