docs: Explain how to route requests from an apphook back to the CMS

[jrief]

Description

This documentation patch explains how to route requests from an apphook back to the CMS. Currently it states that this is impossible, but actually it can be done with a small modification to get_urls()….

Adding CMS pages below an apphooked page was a request from my team btw.

• [x] I have opened this pull request against develop-4
• [ ] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[fsbraun]

Nice!