Start work on genericizing /rustdoc (#13745)

This PR begins the process of making the backing infrastructure for the
`/rustdoc` command more generic such that it can be applied to
additional documentation providers.

In this PR we:

- Rename the `rustdoc` crate to `indexed_docs` as a more general-purpose
name
- Start moving rustdoc-specific functionality into
`indexed_docs::providers::rustdoc`
- Add an `IndexedDocsRegistry` to hold multiple `IndexedDocsStore`s (one
per provider)

We haven't yet removed the rustdoc-specific bits in the `DocsIndexer`.
That will follow soon.

Release Notes:

- N/A

crates/indexed_docs/src/indexed_docs.rs

@@ -0,0 +1,8 @@
+mod indexer;
+mod providers;
+mod registry;
+mod store;
+
+pub use crate::providers::rustdoc::*;
+pub use crate::registry::*;
+pub use crate::store::*;