Add a class docstring to Tree

src/textual/widgets/_tree.py

@@ -305,6 +305,8 @@ class TreeNode(Generic[TreeDataType]):
 
 
 class Tree(Generic[TreeDataType], ScrollView, can_focus=True):
+    """A widget for displaying and navigating data in a tree."""
+
     BINDINGS: ClassVar[list[BindingType]] = [
         Binding("enter", "select_cursor", "Select", show=False),
         Binding("space", "toggle_node", "Toggle", show=False),