packages,plugins: add package documentation

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

plugins/shortcuts/api-report.md

@@ -62,6 +62,4 @@ export const shortcutsApiRef: ApiRef<ShortcutApi>;
 //
 // @public (undocumented)
 export const shortcutsPlugin: BackstagePlugin<{}, {}>;
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/shortcuts/src/index.ts

@@ -13,6 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+/**
+ * A Backstage plugin that provides a shortcuts feature to the sidebar
+ *
+ * @packageDocumentation
+ */
+
 export { shortcutsPlugin, Shortcuts } from './plugin';
 export * from './api';
 export type { Shortcut } from './types';