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/assistant_slash_commands/Cargo.toml

@@ -27,7 +27,6 @@ globset.workspace = true
 gpui.workspace = true
 html_to_markdown.workspace = true
 http_client.workspace = true
-indexed_docs.workspace = true
 language.workspace = true
 project.workspace = true
 prompt_store.workspace = true