website/docs: Add note about single group per role (#12169)

This change adds an admonition to document the fact that every role can only ever be assigned to a single group at the same time. Since this is surprising based on a traditional understanding of role-based models, I've decided to make this a `:::warning`.

I'm undecided on the best place for this information, but for now, decided on putting it into the context of the action that can fail: assigning a role to a group.

While this does not close the issue, it documents this behavior to at least address the "needs documentation" aspect of #10983 .

Signed-off-by: Zuri Klaschka <pklaschka@users.noreply.github.com>

website/docs/users-sources/roles/manage_roles.md

@@ -37,6 +37,10 @@ To delete a role, follow these steps:
 
 In authentik, roles are assigned to [groups](../groups/index.mdx), not to individual users.
 
+:::warning
+In authentik, each role can only be applied to a single group at the moment.
+:::
+
 1.  To assign the role to a group, navigate to **Directory -> Groups**.
 2.  Click the name of the group to which you want to add a role.
 3.  On the group's detail page, on the Overview tab, click **Edit** in the **Group Info** area.