feat(altdoc): evaluate shared gh-pages concurrency group or decoupled deploy step

[dem-extra1]

Context

In PR #654 (closing gha#611), quarto-publish.yml, preview-deploy.yml, and preview-cleanup.yml were updated to share concurrency.group: gh-pages to serialize all writes to the gh-pages branch.

Follow-up Considerations (from PR #654 Claude review)

1. altdoc-multiversion-docs.yml:

   • Currently uses concurrency: group: altdoc-multiversion-docs-${{ github.event_name == 'pull_request' && github.event.pull_request.number || github.ref }} (gha#306/altdoc-multiversion-docs: concurrent PR builds race on the same pr-preview path #311).
   • Writes directly to gh-pages via JamesIves/github-pages-deploy-action / rossjrw/pr-preview-action.
   • Consider separating the build and deploy jobs (like preview.yml + preview-deploy.yml) so the long build doesn't hold the shared gh-pages lock while ensuring deploy steps are serialized with other gh-pages writes.

2. preview-deploy.yml Concurrency Queueing:

   • Clarify the comment in preview-deploy.yml:44-49: GitHub Actions only retains one pending run per concurrency group (superseding older pending runs in the queue), rather than buffering unbounded pending runs.