ci: notify help-docs hub on docs changes

[saravmajestic]

Adds a tiny workflow that, when a PR touching docs merges to the default branch, notifies the help-docs hub (repository_dispatch) so the change is promoted up into help-docs as a review PR.

Part of the bidirectional docs-sync (help-docs is the source-of-truth hub). See tools/DOCS_SYNC.md in help-docs.

Requires the shared docs-sync GitHub App and these Actions settings on this repo (or org-level):

• variable DOCS_SYNC_APP_ID
• secret DOCS_SYNC_APP_PRIVATE_KEY

Inert until those are set. Sync-generated merges (marker [docs-sync]) are skipped, so it never loops.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Added automated notifications when documentation changes are published.
  • Documentation updates now include relevant product and source-change details.
  • Added safeguards to prevent duplicate or unintended documentation notifications.
  • Improved consistency and timeliness when routing documentation changes for review and publication.
  • Added an option to trigger documentation notifications manually when needed.