fix(ci): set baseURL for PR preview subdirectory deployment (#6658)

* refactor(ci): improve PR preview with products.yml and index page

- Load product prefixes from data/products.yml (single source of truth)
- Build regex pattern dynamically from loaded namespaces
- Fix CSS copying: Hugo outputs fingerprinted CSS at root level
- Generate index.html with clickable links to preview pages
- Add test for influxdb3_explorer namespace

This keeps the URL parser in sync as new products are added and
provides a better preview experience with navigable links.

* fix(ci): set baseURL for preview subdirectory deployment

Hugo generates absolute paths for fingerprinted assets based on baseURL.
When deploying to GitHub Pages subdirectory (pr-preview/pr-XXXX/), assets
were incorrectly resolving to the root instead of the preview path.

This sets --baseURL to the correct preview URL so all asset paths work.

.github/workflows/pr-preview.yml

@@ -85,9 +85,12 @@ jobs:
       - name: Build Hugo site
         if: steps.detect.outputs.pages-to-deploy != '[]'
         id: build
+        env:
+          # Set baseURL for GitHub Pages preview subdirectory
+          PREVIEW_BASE_URL: https://influxdata.github.io/docs-v2/pr-preview/pr-${{ github.event.number }}/
         run: |
           START_TIME=$(date +%s)
-          npx hugo --minify
+          npx hugo --minify --baseURL "$PREVIEW_BASE_URL"
           END_TIME=$(date +%s)
           DURATION=$((END_TIME - START_TIME))
           echo "build-time=${DURATION}s" >> $GITHUB_OUTPUT