docs: add info about custom errors via `APIError` to hooks overview (#13047)

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### What?

I was searching for info on how to return custom errors in hooks and
only came upon this discord answer by Dan Ribbons:

https://payloadcms.com/community-help/github/customize-rest-api-response-in-collection-hooks

### Why?

I think this info should be in the docs.

### How?

This PR adds a `Cusom Error Messages` section to the hooks-overview
page, detailing the usage of `APIError` as proposed by Dan Ribbons in
the above discord answer.

Co-authored-by: Paul Popus <paul@payloadcms.com>