fix: docs Pages deploy permissions and document GitHub Pages setup

Deploy job failed because Pages source must be GitHub Actions; document
About/description guidance and fix workflow job permissions.

.github/workflows/docs.yml

@@ -23,6 +23,10 @@ concurrency:
 jobs:
   build:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
     steps:
       - name: Checkout
         uses: actions/checkout@v7
@@ -52,6 +56,9 @@ jobs:
   deploy:
     needs: build
     runs-on: ubuntu-latest
+    permissions:
+      pages: write
+      id-token: write
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}