packages,plugins: add package documentation

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

plugins/code-coverage/api-report.md

@@ -35,6 +35,4 @@ export { isCodeCoverageAvailable as isPluginApplicableToEntity };
 //
 // @public (undocumented)
 export const Router: () => JSX.Element;
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/code-coverage/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 helps you keep track of your code coverage
+ *
+ * @packageDocumentation
+ */
+
 export { codeCoveragePlugin, EntityCodeCoverageContent } from './plugin';
 export {
   Router,