ci: deploy landing page to GitHub Pages

- Rename site/gpp-landing.html -> site/index.html (Pages serves root)
- Add Actions Pages workflow uploading site/ on push to main

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mahabubul470

.github/workflows/pages.yml

@@ -0,0 +1,41 @@
+name: Deploy landing page
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "site/**"
+      - ".github/workflows/pages.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Allow one concurrent deployment; newer pushes supersede in-flight runs.
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Configure Pages
+        uses: actions/configure-pages@v5
+
+      - name: Upload site/ as Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: site
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4