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/src/assistant.rs

@@ -20,9 +20,9 @@ pub(crate) use completion_provider::*;
 pub(crate) use context_store::*;
 use fs::Fs;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
+use indexed_docs::{IndexedDocsRegistry, Provider};
 pub(crate) use inline_assistant::*;
 pub(crate) use model_selector::*;
-use rustdoc::RustdocStore;
 use semantic_index::{CloudEmbeddingProvider, SemanticIndex};
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};
@@ -292,7 +292,8 @@ pub fn init(fs: Arc<dyn Fs>, client: Arc<Client>, cx: &mut AppContext) {
     assistant_panel::init(cx);
     inline_assistant::init(fs.clone(), client.telemetry().clone(), cx);
     terminal_inline_assistant::init(fs.clone(), client.telemetry().clone(), cx);
-    RustdocStore::init_global(cx);
+    IndexedDocsRegistry::init_global(cx);
+    register_indexed_docs_providers(cx);
 
     CommandPaletteFilter::update_global(cx, |filter, _cx| {
         filter.hide_namespace(Assistant::NAMESPACE);
@@ -327,6 +328,12 @@ fn register_slash_commands(cx: &mut AppContext) {
     slash_command_registry.register_command(fetch_command::FetchSlashCommand, false);
 }
 
+fn register_indexed_docs_providers(cx: &mut AppContext) {
+    let indexed_docs_registry = IndexedDocsRegistry::global(cx);
+
+    indexed_docs_registry.register_provider(Provider::rustdoc());
+}
+
 pub fn humanize_token_count(count: usize) -> String {
     match count {
         0..=999 => count.to_string(),