packages,plugins: add package documentation

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

plugins/allure/api-report.md

@@ -22,6 +22,4 @@ export const allurePlugin: BackstagePlugin<
 //
 // @public (undocumented)
 export const EntityAllureReportContent: () => JSX.Element;
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/allure/src/index.ts

@@ -13,4 +13,11 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+/**
+ * A Backstage plugin that integrates with Allure
+ *
+ * @packageDocumentation
+ */
+
 export { allurePlugin, EntityAllureReportContent } from './plugin';