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/assistant/Cargo.toml

@@ -13,8 +13,8 @@ path = "src/assistant.rs"
 doctest = false
 
 [dependencies]
-anyhow.workspace = true
 anthropic = { workspace = true, features = ["schemars"] }
+anyhow.workspace = true
 assistant_slash_command.workspace = true
 async-watch.workspace = true
 cargo_toml.workspace = true
@@ -32,6 +32,7 @@ gpui.workspace = true
 heed.workspace = true
 html_to_markdown.workspace = true
 http.workspace = true
+indexed_docs.workspace = true
 indoc.workspace = true
 language.workspace = true
 log.workspace = true
@@ -45,7 +46,6 @@ paths.workspace = true
 project.workspace = true
 regex.workspace = true
 rope.workspace = true
-rustdoc.workspace = true
 schemars.workspace = true
 search.workspace = true
 semantic_index.workspace = true