assistant: Show a warning indicator when the user needs to run `cargo doc` (#14262)

This PR updates the `/docs` slash command to show a warning to the user
if a crate's docs cannot be indexed due to the target directory not
containing docs:

<img width="782" alt="Screenshot 2024-07-11 at 5 11 46 PM"
src="https://github.com/user-attachments/assets/2f54f7a1-97f4-4d2d-b51f-57ba31e50a2f">

Release Notes:

- N/A