packages,plugins: add package documentation

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

plugins/app-backend/api-report.md

@@ -24,6 +24,4 @@ export interface RouterOptions {
   logger: Logger_2;
   staticFallbackHandler?: express.Handler;
 }
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/app-backend/src/index.ts

@@ -14,4 +14,10 @@
  * limitations under the License.
  */
 
+/**
+ * A Backstage backend plugin that serves the Backstage frontend app
+ *
+ * @packageDocumentation
+ */
+
 export * from './service/router';