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/registry.rs

@@ -0,0 +1,47 @@
+use std::sync::Arc;
+
+use collections::HashMap;
+use gpui::{AppContext, BackgroundExecutor, Global, ReadGlobal, UpdateGlobal};
+use parking_lot::RwLock;
+
+use crate::{IndexedDocsStore, Provider, ProviderId};
+
+struct GlobalIndexedDocsRegistry(Arc<IndexedDocsRegistry>);
+
+impl Global for GlobalIndexedDocsRegistry {}
+
+pub struct IndexedDocsRegistry {
+    executor: BackgroundExecutor,
+    stores_by_provider: RwLock<HashMap<ProviderId, Arc<IndexedDocsStore>>>,
+}
+
+impl IndexedDocsRegistry {
+    pub fn global(cx: &AppContext) -> Arc<Self> {
+        GlobalIndexedDocsRegistry::global(cx).0.clone()
+    }
+
+    pub fn init_global(cx: &mut AppContext) {
+        GlobalIndexedDocsRegistry::set_global(
+            cx,
+            GlobalIndexedDocsRegistry(Arc::new(Self::new(cx.background_executor().clone()))),
+        );
+    }
+
+    pub fn new(executor: BackgroundExecutor) -> Self {
+        Self {
+            executor,
+            stores_by_provider: RwLock::new(HashMap::default()),
+        }
+    }
+
+    pub fn register_provider(&self, provider: Provider) {
+        self.stores_by_provider.write().insert(
+            provider.id.clone(),
+            Arc::new(IndexedDocsStore::new(provider, self.executor.clone())),
+        );
+    }
+
+    pub fn get_provider_store(&self, provider_id: ProviderId) -> Option<Arc<IndexedDocsStore>> {
+        self.stores_by_provider.read().get(&provider_id).cloned()
+    }
+}