packages,plugins: add package documentation

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

plugins/cloudbuild/api-report.md

@@ -361,6 +361,4 @@ export interface Volume {
   // (undocumented)
   path: string;
 }
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/cloudbuild/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 integrates towards Google Cloud Build
+ *
+ * @packageDocumentation
+ */
+
 export {
   cloudbuildPlugin,
   cloudbuildPlugin as plugin,