assistant: Populate /docs rustdoc with workspace crates (#16172)

This PR makes the `/docs` slash command populate the list with all of
the workspace crates when using the local rustdoc provider.

The workspace crates are shown in the search results when a crate is not
already indexed:

<img width="577" alt="Screenshot 2024-08-13 at 2 18 39 PM"
src="https://github.com/user-attachments/assets/39bee576-8e1a-4b21-a9f8-7951ebae4cc3">

These crates are shown with an `(unindexed)` suffix to convey this:

<img width="570" alt="Screenshot 2024-08-13 at 2 18 45 PM"
src="https://github.com/user-attachments/assets/4eeb07f7-378f-44d4-ae11-4ffe45a23964">

Release Notes:

- N/A

crates/indexed_docs/Cargo.toml

@@ -14,6 +14,7 @@ path = "src/indexed_docs.rs"
 [dependencies]
 anyhow.workspace = true
 async-trait.workspace = true
+cargo_metadata.workspace = true
 collections.workspace = true
 derive_more.workspace = true
 fs.workspace = true