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_host/benches/extension_compilation_benchmark.rs

@@ -132,7 +132,6 @@ fn manifest() -> ExtensionManifest {
             .collect(),
         context_servers: BTreeMap::default(),
         slash_commands: BTreeMap::default(),
-        indexed_docs_providers: BTreeMap::default(),
         snippets: None,
         capabilities: vec![ExtensionCapability::ProcessExec(
             extension::ProcessExecCapability {