docs: render missing docs for scheduled tasks admin api

[dklimpel]

The docs were created in:

• Add an Admin API endpoint to fetch scheduled tasks  #18214

It will have been an oversight that the docs were not added to the SUMMARY.md (TOC).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[CLAassistant]

All committers have signed the CLA.

[anoadragon453]

Thanks for doing this ❤️