packages,plugins: add package documentation

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

plugins/bitrise/api-report.md

@@ -22,6 +22,4 @@ export const EntityBitriseContent: () => JSX.Element;
 //
 // @public (undocumented)
 export const isBitriseAvailable: (entity: Entity) => boolean;
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/bitrise/src/index.ts

@@ -14,6 +14,12 @@
  * limitations under the License.
  */
 
+/**
+ * A Backstage plugin that integrates towards Bitrise
+ *
+ * @packageDocumentation
+ */
+
 export { bitrisePlugin } from './plugin';
 export { EntityBitriseContent } from './extensions';
 export { isBitriseAvailable } from './components/BitriseBuildsComponent';