Document feature_breadcrumbs_v2 (#19700)

Author: t3chguy

docs/labs.md

@@ -154,3 +154,8 @@ entirely incomplete and may not work at all - it is not recommended for general
 
 Metaspaces are automatically populated spaces you can enable in your Space panel.
 By default, you'll have Home or All rooms, but you can opt in to a People, Favourites, and Other Rooms metaspace too.
+
+## Breadcrumbs v2 (`feature_breadcrumbs_v2`)
+
+Instead of showing the horizontal list of breadcrumbs under the filter field, the new UX is an interactive context menu
+triggered by the button to the right of the filter field.