Run the docs check when `CHANGELOG.md` changes

`CHANGELOG.md` is the source of truth for the generated Open source
changelog page, and `CHANGELOGS_CHECK_TRIGGERS` already lists it, but it
was missing from the `docs_job_mintlify` digest, so a pull request that
touched only `CHANGELOG.md` never scheduled the job and that inner
trigger could not fire. This is how the stale page reached `master`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>