docs: add visual header explaining nested scrolling in LazyColumn

Add a visual header to each map card in MapsInLazyColumnActivity to
explicitly demonstrate how the map interaction disables the list's
scrolling. This provides a clear, in-app reference for developers
facing issues with nested scrolling in LazyColumn.

Fixes #14

Author: kikoso

maps-app/src/main/java/com/google/maps/android/compose/MapsInLazyColumnActivity.kt

@@ -289,6 +289,15 @@ private fun MapCard(
                 )
             }
 
+            Column(
+                modifier = Modifier.align(Alignment.TopStart)
+            ) {
+                TextWithBackground(
+                    "Panning this map disables list scroll",
+                    fontWeight = FontWeight.Bold
+                )
+            }
+
             Column(
                 modifier = Modifier.align(Alignment.BottomStart)
             ) {