Put /docs behind a feature flag (#16000)

This PR puts the `/docs` slash command behind a feature flag.

Release Notes:

- N/A

crates/assistant/src/slash_command/docs_command.rs

@@ -7,6 +7,7 @@ use anyhow::{anyhow, bail, Result};
 use assistant_slash_command::{
     ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
 };
+use feature_flags::FeatureFlag;
 use gpui::{AppContext, BackgroundExecutor, Model, Task, WeakView};
 use indexed_docs::{
     DocsDotRsProvider, IndexedDocsRegistry, IndexedDocsStore, LocalRustdocProvider, PackageName,
@@ -18,6 +19,12 @@ use ui::prelude::*;
 use util::{maybe, ResultExt};
 use workspace::Workspace;
 
+pub(crate) struct DocsSlashCommandFeatureFlag;
+
+impl FeatureFlag for DocsSlashCommandFeatureFlag {
+    const NAME: &'static str = "docs-slash-command";
+}
+
 pub(crate) struct DocsSlashCommand;
 
 impl DocsSlashCommand {