Issue @deprecated notice for the UnderlineNav v1

.changeset/dull-lies-count.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': minor
+---
+
+Issue a deprecation notice for UnderlineNav v1

docs/content/UnderlineNav.mdx

@@ -8,6 +8,10 @@ import data from '../../src/UnderlineNav.docs.json'
 
 Use the UnderlineNav component to style navigation with a minimal underlined selected state, typically used for navigation placed at the top of the page.
 
+## Deprecation
+
+UnderlineNav is deprecated and will be replaced by the draft `UnderlineNav` in the next major release. See [the draft UnderlineNav's docs](/drafts/UnderlineNav2) for more details.
+
 To use UnderlineNav with [react-router](https://github.com/ReactTraining/react-router) or
 [react-router-dom](https://www.npmjs.com/package/react-router-dom), pass
 `as={NavLink}` and omit the `selected` prop.

src/UnderlineNav.tsx

@@ -111,4 +111,7 @@ const UnderlineNavLink = styled.a.attrs<StyledUnderlineNavLinkProps>(props => ({
 UnderlineNavLink.displayName = 'UnderlineNav.Link'
 
 export type UnderlineNavLinkProps = ComponentProps<typeof UnderlineNavLink>
+/**
+ * @deprecated UnderlineNav is deprecated and will be replaced by the draft `UnderlineNav` in the next major release. See https://primer.style/react/drafts/UnderlineNav2 for more details.
+ */
 export default Object.assign(UnderlineNav, {Link: UnderlineNavLink})