Document that InputGroup can be used inside FormLayout (#713)

The InputGroup component already inherits its layout from FormLayout
when nested inside it, but the docs did not mention this. Also add
InputGroup to FormLayout's list of supported form fields, since its
own docs already demonstrate nesting InputGroup.

Author: github-actions[bot]

src/components/FormLayout/README.md

@@ -49,7 +49,8 @@ there are longer validation messages or help texts.
 
 The FormLayout supports buttons and all React UI form fields:
 [Button](/components/Button), [CheckboxField](/components/CheckboxField),
-[FileInputField](/components/FileInputField), [Radio](/components/Radio),
+[FileInputField](/components/FileInputField),
+[InputGroup](/components/InputGroup), [Radio](/components/Radio),
 [SelectField](/components/SelectField), [TextArea](/components/TextArea),
 [TextField](/components/TextField), and [Toggle](/components/Toggle).
 

src/components/InputGroup/README.md

@@ -73,6 +73,10 @@ See [API](#api) for all available options.
   [ButtonGroup](/components/ButtonGroup) component which is designed
   specifically for that purpose.
 
+- InputGroup can be used inside [FormLayout](/components/FormLayout). In that
+  case, its `layout` prop is ignored and the value is inherited from
+  FormLayout instead.
+
 ## Sizes
 
 All existing field and button sizes are also available on the input group level: