Docs: add badge to feature available for admins only

doc/administration/custom_hooks.md

-# Custom server-side Git hooks
+# Custom server-side Git hooks **(CORE ONLY)**
 
 NOTE: **Note:**
 Custom Git hooks must be configured on the filesystem of the GitLab

doc/administration/gitaly/index.md

@@ -334,7 +334,10 @@ When you tail the Gitaly logs on your Gitaly server you should see requests
 coming in. One sure way to trigger a Gitaly request is to clone a repository
 from your GitLab server over HTTP.
 
-DANGER: **Danger:** If you have [custom server-side Git hooks](../custom_hooks.md#custom-server-side-git-hooks) configured, either per repository or globally, you must move these to the Gitaly node. If you have multiple Gitaly nodes, copy your custom hook(s) to all nodes.
+DANGER: **Danger:**
+If you have [custom server-side Git hooks](../custom_hooks.md) configured,
+either per repository or globally, you must move these to the Gitaly node.
+If you have multiple Gitaly nodes, copy your custom hook(s) to all nodes.
 
 ### Disabling the Gitaly service in a cluster environment