Fix deployment of docs to gh-pages on release, blocked by PR previews

[adamkudrna]

Summary

• mkdocs gh-deploy always builds a parentless commit for gh-pages (the job's checkout only fetches master, so it has no local history for gh-pages to branch from), which needs --force to push. --force was removed when PR previews (pr-preview/ on gh-pages, see pull-request-preview.yml) were introduced, to avoid wiping them — but that also means the push is now rejected as a non-fast-forward every time, so no release docs have deployed since v0.63.1.
• Replace mkdocs gh-deploy with mkdocs build plus a manual deploy step: check out the real gh-pages tip into a worktree, replace everything except pr-preview/ with the freshly built site, add .nojekyll, commit, and push normally (a true fast-forward, no force needed).
• Commit message for the deploy now references the React UI release version and source SHA.

Test plan

• npm run build passes
• mkdocs build passes
• npm run lint passes
• npm run test:jest passes
• npm run test:playwright-ct:all passes
• Reproduced the original failure and verified the fix against a throwaway git remote mirroring the real gh-pages state (preserves pr-preview/, deploys new content, no-ops when content is unchanged)
• Confirm deploy_docs succeeds on the next real release