fix(ci): serialize docs deployments to gh-pages

Concurrent pushes to main started overlapping docs builds; the last to finish was rejected as non-fast-forward, and an older commit could overwrite newer docs. Pull request previews never push and stay unserialized.

Author: cmgzn

.github/workflows/deploy_sphinx_docs.yml

@@ -21,6 +21,11 @@ on:
         type: boolean
         default: false
 
+# Serialize gh-pages deployments so concurrent runs cannot push from a stale branch.
+concurrency:
+  group: ${{ github.event_name == 'pull_request' && format('docs-preview-{0}', github.run_id) || 'docs-deploy-gh-pages' }}
+  cancel-in-progress: false
+
 jobs:
   pages:
     runs-on: ubuntu-latest