Ensure docs are build for version branches

This is needed to start onboarding `9.0` to the documentation build

elastic/docs-builder#1180

Author: Mpdreamz

.github/workflows/docs-build.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - main
+      - '\d+.\d+'
   pull_request_target: ~
   merge_group: ~