packages,plugins: add package documentation

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

plugins/proxy-backend/api-report.md

@@ -13,6 +13,4 @@ import { PluginEndpointDiscovery } from '@backstage/backend-common';
 //
 // @public (undocumented)
 export function createRouter(options: RouterOptions): Promise<express.Router>;
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/proxy-backend/src/index.ts

@@ -14,4 +14,10 @@
  * limitations under the License.
  */
 
+/**
+ * A Backstage backend plugin that helps you set up proxy endpoints in the backend
+ *
+ * @packageDocumentation
+ */
+
 export * from './service';