ci: rename page workflow, and build doc by nix

Signed-off-by: Burgess Chang <[email protected]>

.github/workflows/pages.yaml

@@ -0,0 +1,48 @@
+name: pages
+
+on:
+  push:
+    branches:
+      - main
+  workflow_call:
+  workflow_dispatch:
+
+concurrency:
+  group: pages
+
+jobs:
+  build:
+    name: build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install nix
+        uses: cachix/install-nix-action@v25
+
+      - name: Setup pages
+        uses: actions/configure-pages@v4
+
+      - name: Build doc
+        run: |
+          nix build ./doc
+          cp -r --no-preserve=all result/ _site
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+
+  deploy:
+    name: deploy
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    permissions:
+      id-token: write
+      pages: write
+    steps:
+      - name: Deploy pages
+        id: deployment
+        uses: actions/deploy-pages@v4