[CI] Serialize api doc workflow

[hughperkins]

Issue: #

Brief Summary

copilot:summary

Walkthrough

copilot:walkthrough

[claude]

LGTM — simple job-level concurrency addition to serialize gh-pages deployments across refs.

Extended reasoning...

Overview

Single change to .github/workflows/api_doc.yml: adds a concurrency.group: gh-pages-deploy block at the job level. This serializes the deploy job across all workflow runs regardless of ref, complementing the existing workflow-level concurrency that cancels same-ref duplicates.

Security risks

None. This is a CI configuration change with no access to secrets beyond the existing GITHUB_TOKEN usage, no new permissions, and no logic changes.

Level of scrutiny

Low. The change is mechanical and the intent is clear: prevent concurrent gh-pages deployments from racing. The fixed group name (no cancel-in-progress) means jobs queue rather than cancel, which is appropriate for serialization semantics.

Other factors

No bugs found, no prior reviews, clean diff touching only one CI file.