assistant: Add the "create your command" item (#16467)

This PR adds an extra item to the slash command picker that links users to the doc that teaches how to create a custom one.

Release Notes:

- N/A

---------

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>

crates/picker/src/picker.rs

@@ -524,7 +524,7 @@ impl<D: PickerDelegate> Picker<D> {
                     picker
                         .border_color(cx.theme().colors().border_variant)
                         .border_b_1()
-                        .pb(px(-1.0))
+                        .py(px(-1.0))
                 },
             )
     }