assistant: Auto-suggest crates for `/docs` when using the docs.rs provider (#16169)

This PR improves the initial experience of using `/docs docs-rs` with an
empty index.

We now show a brief explainer of what is expected (a crate name) as well
as list some popular Rust crates to try:

<img width="540" alt="Screenshot 2024-08-13 at 12 25 39 PM"
src="https://github.com/user-attachments/assets/942de250-2901-45df-9e3e-52ff3b3fc517">

Release Notes:

- N/A