changesets: added changeset for SubRoutes

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/heavy-lemons-hope.md

@@ -0,0 +1,41 @@
+---
+'@backstage/core-api': patch
+---
+
+Add `SubRouteRef`s, which can be used to create a route ref with a fixed path relative to an absolute `RouteRef`. They are useful if you for example have a page that is mounted at a sub route of a routable extension component, and you want other plugins to be able to route to that page.
+
+For example:
+
+```tsx
+// routes.ts
+const rootRouteRef = createRouteRef({ id: 'root' });
+const detailsRouteRef = createSubRouteRef({
+  id: 'root-sub',
+  parent: rootRouteRef,
+  path: '/details',
+});
+
+// plugin.ts
+export const myPlugin = createPlugin({
+  routes: {
+    root: rootRouteRef,
+    details: detailsRouteRef,
+  }
+})
+
+export const MyPage = plugin.provide(createRoutableExtension({
+  component: () => import('./components/MyPage').then(m => m.MyPage),
+  mountPoint: rootRouteRef,
+}))
+
+// components/MyPage.tsx
+const MyPage = () => (
+  <Routes>
+    {/* myPlugin.routes.root will take the user to this page */}
+    <Route path='/' element={<IndexPage />}>
+
+    {/* myPlugin.routes.details will take the user to this page */}
+    <Route path='/details' element={<DetailsPage />}>
+  </Routes>
+)
+```