assistant: Add MVP for /rustdoc using indexed docs (#12952)

This PR adds an MVP of retrieving docs using the `/rustdoc` command from
an indexed set of docs.

To try this out:

1. Build local docs using `cargo doc`
2. Index the docs for the crate you want to search using `/rustdoc
--index <CRATE_NAME>`
    - Note: This may take a while, depending on the size of the crate
3. Search for docs using `/rustdoc my_crate::path::to::item`
    - You should get completions for the available items

Here are some screenshots of it in action:

<img width="640" alt="Screenshot 2024-06-12 at 6 19 20 PM"
src="https://github.com/zed-industries/zed/assets/1486634/6c49bec9-d084-4dcb-a92c-1b4c557ee9ce">

<img width="636" alt="Screenshot 2024-06-12 at 6 52 56 PM"
src="https://github.com/zed-industries/zed/assets/1486634/636a651c-7d02-48dc-b05c-931f33c49f9c">

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -21,6 +21,7 @@ pub(crate) use context_store::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
 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};
@@ -286,6 +287,7 @@ pub fn init(client: Arc<Client>, cx: &mut AppContext) {
     register_slash_commands(cx);
     assistant_panel::init(cx);
     inline_assistant::init(client.telemetry().clone(), cx);
+    RustdocStore::init_global(cx);
 
     CommandPaletteFilter::update_global(cx, |filter, _cx| {
         filter.hide_namespace(Assistant::NAMESPACE);