Add Flex widget documentation

docs/source/reference/lua-scripting/user_interface.rst

@@ -81,6 +81,7 @@ Widget types
    Text: Displays text. <widgets/text>
    TextEdit: Accepts text input from the user. <widgets/textedit>
    Image: Renders a texture. <widgets/image>
+   Flex: Aligns children in a column/row <widgets/flex>
 
 Example
 -------

docs/source/reference/lua-scripting/widgets/flex.rst

@@ -0,0 +1,43 @@
+Flex Widget
+===========
+
+Aligns its children along either a column or a row, depending on the `horizontal` property.
+
+Properties
+----------
+
+.. list-table::
+  :header-rows: 1
+  :widths: 20 20 60
+
+  * - name
+    - type (default value)
+    - description
+  * - horizontal
+    - bool (false)
+    - Flex aligns its children in a row if true, otherwise in a column.
+  * - autoSize
+    - bool (true)
+    - | If true, Flex will automatically resize to fit its contents.
+      | Children can't be relatively position/sized when true.
+  * - align
+    - ui.ALIGNMENT (Start)
+    - Where to align the children in the main axis.
+  * - arrange
+    - ui.ALIGNMETN (Start)
+    - How to arrange the children in the cross axis.
+
+External
+--------
+.. list-table::
+  :header-rows: 1
+  :widths: 20 20 60
+
+  * - name
+    - type (default value)
+    - description
+  * - grow
+    - float (0)
+    - | Grow factor for the child. If there is unused space in the Flex,
+      | it will be split between widgets according to this value.
+      | Has no effect if `autoSize` is `true`.