Sync classref with current source

Changes order of brief/long descriptions and removes category
as per godotengine/godot#35132.

classes/class_boxcontainer.rst

@@ -13,13 +13,13 @@ BoxContainer
 
 **Inherited By:** :ref:`ColorPicker<class_ColorPicker>`, :ref:`HBoxContainer<class_HBoxContainer>`, :ref:`VBoxContainer<class_VBoxContainer>`
 
-**Category:** Core
-
-Brief Description
------------------
-
 Base class for box containers.
 
+Description
+-----------
+
+Arranges child controls vertically or horizontally, and rearranges the controls automatically when their minimum size changes.
+
 Properties
 ----------
 
@@ -55,11 +55,6 @@ enum **AlignMode**:
 
 - **ALIGN_END** = **2** --- Aligns children with the end of the container.
 
-Description
------------
-
-Arranges child controls vertically or horizontally, and rearranges the controls automatically when their minimum size changes.
-
 Property Descriptions
 ---------------------