ci: tweak docs preview url

Author: robtaylor

.github/workflows/preview-docs.yml

@@ -13,8 +13,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - name: Set base URL for preview if PR
-        run: echo "DOCS_BASE_URL=https://${{ env.DOMAIN }}/${{ env.PR_PATH}}/" >> $GITHUB_ENV
 
       - name: Setup PDM
         uses: pdm-project/setup-pdm@v4
@@ -31,7 +29,8 @@ jobs:
 
       - uses: rossjrw/pr-preview-action@v1
         with:
-          source-dir: .
+          source-dir: docs/_build
           preview-branch: gh-pages
           umbrella-dir: pr-preview
           action: auto
+          pages-base-url: docs.chipflow-infra.com/chipflow-lib