docs: Add x.ai Grok example (#21655)

- Closes https://github.com/zed-industries/zed/issues/21635

<img width="639" alt="Screenshot 2024-12-06 at 13 57 42"
src="https://github.com/user-attachments/assets/a4434edb-3c7d-40c0-9df8-7e928a9307d0">


Release Notes:

- Document support for x.ai Grok