assistant: Add /docs slash command (#13794)

This PR adds a new `/docs` slash command to the Assistant. This slash
command replaces `/rustdoc`.

The `/docs` slash command works with different providers. There is
currently a built-in provider for rustdoc, but new providers can be
defined within extensions. The Gleam extension contains an example of
this.

When you first type `/docs` a completion menu will be shown with the
list of available providers:


https://github.com/zed-industries/zed/assets/1486634/32287000-5855-44d9-a2eb-569596f5abd9

After completing the provider you want to use then you can type the
package name and/or item path to search for the relevant docs:


https://github.com/zed-industries/zed/assets/1486634/6fc55a63-7fcd-42ea-80ce-08c670bf03fc

There are still some rough edges around completions that I would like to
get cleaned up in a future PR. Both of these seem to stem from the fact
that we're using an intermediate completion in the slash command:

1. Accepting a provider completion will show an error until you press
<kbd>Space</kbd> to continue typing.
- We need a way of not submitting a slash command when a completion is
accepted.
2. We currently need to show the provider name in the documentation item
completion list.
- Without it, the provider name gets wiped out when accepting a
completion, causing the slash command to become invalid.

Release Notes:

- N/A

crates/indexed_docs/src/registry.rs

@@ -34,6 +34,14 @@ impl IndexedDocsRegistry {
         }
     }
 
+    pub fn list_providers(&self) -> Vec<ProviderId> {
+        self.stores_by_provider
+            .read()
+            .keys()
+            .cloned()
+            .collect::<Vec<_>>()
+    }
+
     pub fn register_provider(
         &self,
         provider: Box<dyn IndexedDocsProvider + Send + Sync + 'static>,