HaikuBook: Add documentation for BCardLayout

This adds documentation for BCardView, BCardLayout and BLayoutBuilder::Cards.
There is also a bit of cleanup for the BSplitView documentation.

It also makes explicit when a developer passes an invalid argument to
BCardLayout::SetVisibleItem(), by making that a debugger() call.

Change-Id: I17ac52cc773bb76c4f81beaa76f72af62a9e10f4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2460
Reviewed-by: waddlesplash <[email protected]>

docs/user/interface/View.dox

@@ -4351,6 +4351,9 @@ SetViewColor(Parent()->ViewColor());
 	\fn status_t BView::Perform(perform_code code, void* _data)
 	\brief Perform some action. (Internal Method)
 
+	This method is available to allow classes to be extended while maintaining
+	binary compatibility.
+
 	The following perform codes are recognized:
 		- \c PERFORM_CODE_MIN_SIZE:
 		- \c PERFORM_CODE_MAX_SIZE: