docs: Update SubmitButton component

Change loading label and add to documentation.

src/form/components/SubmitButton.md

@@ -0,0 +1,12 @@
+SubmitButton is a component that renders a button with different states based on
+the `state` prop. It can be used to indicate the status of a form submission.
+
+```jsx padded
+<SubmitButton state={1} />
+
+<SubmitButton state={2} />
+
+<SubmitButton state={3} />
+
+<SubmitButton label="Submitting" state={2} />
+```