Replace Vercel with GitHub pages

This requires additional changes in the repository GitHub Pages
settings. _Source_ needs to be set to _GitHub Actions_, and a custom
domain needs to be configured.

This doesn’t support redirects.

.github/workflows/website.yml

@@ -0,0 +1,25 @@
+name: website
+on:
+  push:
+    branches: [main]
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
+    environment:
+      name: Website
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v3
+        with: {node-version: 18}
+      - run: npm ci
+      - run: npm run docs
+      - uses: actions/upload-pages-artifact@v2
+        with:
+          path: public
+      - uses: actions/deploy-pages@v2
+        id: deployment