Update Antora preview playbook to use adding-anywhere-footnote branch

Author: RayOffiah

antora-playbook.preview.yml

@@ -37,7 +37,7 @@ content:
   ## out and replaced with a non git@ prefix. To revert if it causes issues
   sources:
   - url: .
-    branches: HEAD
+    branches: [adding-anywhere-footnote]
     start_path: home
   - url: https://github.com/couchbaselabs/docs-style-guide
     branches: main