fix(board): restore container grid geometry (#6808)

apps/docs/docs/management/boards/index.mdx

@@ -36,6 +36,11 @@ current Base layout.
 A **Container** groups apps, widgets, and nested Containers. It can have a label, collapse control, app-opening action,
 border color, and CSS classes.
 
+Items can occupy the outermost rows and columns of a Container. The drop preview shows the position where the item
+will remain when released.
+Dragging an item onto another item of the same size swaps their positions. Displaced items snap directly to their
+previewed positions without a movement animation and stay there when you release the pointer.
+
 A **rail** reserves one to three columns at the left or right of a non-Mobile layout. Rails stay fixed while the page
 scrolls and store their contents separately for each layout.