Update custom_hooks.md for chained hooks support
What does this MR do?
Add documentation for gitlab-shell!93 (closed)
Are there points in the code the reviewer needs to double check?
Why was this MR needed?
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
- CHANGELOG entry added
- Documentation created/updated
- API support added
- Added for this feature/bug
- All builds are passing
- Conform by the merge request performance guides
- Conform by the style guides
Branch has no merge conflicts with
master(if you do - rebase it please)
- Squashed related commits together