feat: deploy PR preview to gh pages

Author: mattwang44

.github/workflows/ci.yml

@@ -1,11 +1,15 @@
 name: build
+concurrency: preview-${{ github.ref }}
+
+permissions:
+  contents: write
 
 on:
   pull_request:
 
 jobs:
   ci:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
@@ -19,5 +23,13 @@ jobs:
       - name: Install uv
         uses: astral-sh/setup-uv@v6
 
-      - name: Validate
+      - name: Build HTML Docs
         run: VERSION=${{ github.event.repository.default_branch }} JOBS=4 MODE=html make all
+
+      - name: Deploy PR Doc Preview
+        uses: rossjrw/pr-preview-action@v1
+        with:
+          source-dir: ../cpython/Doc/build/html
+          preview-branch: gh-pages
+          umbrella-dir: pr-preview
+          action: auto