docs: update channel search cookbook (#2363)

Updated cookbook for Channel Search. It includes:

- Overriding search item preview
- Rolling your own search, including message search which was not
covered before

Preview here:
https://staging.getstream.io/chat/docs/sdk/react/guides/customization/channel_search/

## To-Do

- [x] Inline screenshots
- [x] Add tabs

Author: myandrienko