Promote package suggestions to a first-class concept on IndexedDocsProviders (#16177)

This PR promotes package suggestions to a first-class concept on the
`IndexedDocsProvider` trait.

This will allow any implementer of `IndexedDocsProvider` to provide a
list of package names to suggest for use with `/docs`.

For the docs.rs provider we use the 250 most popular Rust crates (as
identified [here](https://lib.rs/std)), and for the rustdoc provider we
use the packages in the Cargo workspace.

Release Notes:

- N/A

typos.toml

@@ -12,6 +12,9 @@ extend-exclude = [
     "crates/google_ai/src/supported_countries.rs",
     "crates/open_ai/src/supported_countries.rs",
 
+    # Some crate names are flagged as typos.
+    "crates/indexed_docs/src/providers/rustdoc/popular_crates.txt",
+
     # Stripe IDs are flagged as typos.
     "crates/collab/src/db/tests/processed_stripe_event_tests.rs",
     # Not our typos