Remove /docs slash command (#36325)

This PR removes the `/docs` slash command.

We never fully shipped this—with it requiring explicit opt-in via a
setting—and it doesn't seem like the feature is needed in an agentic
world.

Release Notes:

- Removed the `/docs` slash command.

crates/extension_cli/src/main.rs

@@ -144,10 +144,6 @@ fn extension_provides(manifest: &ExtensionManifest) -> BTreeSet<ExtensionProvide
         provides.insert(ExtensionProvides::ContextServers);
     }
 
-    if !manifest.indexed_docs_providers.is_empty() {
-        provides.insert(ExtensionProvides::IndexedDocsProviders);
-    }
-
     if manifest.snippets.is_some() {
         provides.insert(ExtensionProvides::Snippets);
     }