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/agent_ui/src/agent_configuration.rs

@@ -1035,7 +1035,6 @@ fn extension_only_provides_context_server(manifest: &ExtensionManifest) -> bool
         && manifest.grammars.is_empty()
         && manifest.language_servers.is_empty()
         && manifest.slash_commands.is_empty()
-        && manifest.indexed_docs_providers.is_empty()
         && manifest.snippets.is_none()
         && manifest.debug_locators.is_empty()
 }