Flatten `General` and `Assistant` navigation in docs (#16885)

This PR flattens out the docs nav, so sections like General and
Assistant have a single level of navigation items.

Also renames the `Assistant` page -> `Overview` to be more consistent
with other sections.

| Before | After |
|--------|-------|
| ![CleanShot 2024-08-26 at 11 23
28@2x](https://github.com/user-attachments/assets/06fb9e46-8667-457e-b187-3c2ce2b60369)
| ![CleanShot 2024-08-26 at 11 23
01@2x](https://github.com/user-attachments/assets/1173d75a-53d1-435b-8d1a-c37f28a363d4)
|


Release Notes:

- N/A