ci(pages): replace legacy branch Pages with Actions workflow

Legacy branch-based Pages (source = main /docs with Jekyll) was stuck:
push f63f468 -> 056c71f never triggered a rebuild (last successful build
2026-05-24), and a manual POST .../pages/builds stalled at "building"
for 3+ minutes (normal: 30-40s).

Switch to the modern Pages workflow:
- upload docs/ as artifact via actions/upload-pages-artifact@v3
- deploy via actions/deploy-pages@v4
- triggers on push to main when docs/** or this workflow change
- concurrency group "pages" with cancel-in-progress to keep latest

Pages source already PATCHed to build_type=workflow via gh api.

Author: wanshuiyin

.github/workflows/pages.yml

@@ -0,0 +1,48 @@
+name: Deploy GitHub Pages
+
+# Replaces the legacy branch-based Pages build (was: source = main /docs).
+# This workflow uploads docs/ as a Pages artifact and deploys via the
+# official actions/deploy-pages action. Faster and more reliable than the
+# legacy Jekyll path, especially when the docs/ tree contains many large
+# pre-rendered HTML files.
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "docs/**"
+      - ".github/workflows/pages.yml"
+  workflow_dispatch:
+
+# Allow GITHUB_TOKEN to deploy to Pages.
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Only one Pages deployment at a time; cancel in-progress runs on new push.
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Pages
+        uses: actions/configure-pages@v5
+
+      - name: Upload docs/ as Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: docs
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4