fix: build output location for prs

Author: jeffabailey

.github/workflows/mdbook.yml

@@ -153,7 +153,7 @@ jobs:
         with:
           source-dir: ./book/html
           preview-branch: gh-pages
-          umbrella-dir: pr-preview
+          umbrella-dir: docs/pr-preview
 
   gqm-preview:
     if: github.event_name == 'pull_request' && github.event.action != 'closed' && github.event.pull_request.head.repo.full_name == github.repository