packages,plugins: add package documentation

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

plugins/splunk-on-call/api-report.md

@@ -104,6 +104,4 @@ export class UnauthorizedError extends Error {}
 // Warnings were encountered during analysis:
 //
 // src/plugin.d.ts:7:5 - (ae-forgotten-export) The symbol "SplunkOnCallPageProps" needs to be exported by the entry point index.d.ts
-
-// (No @packageDocumentation comment for this package)
 ```

plugins/splunk-on-call/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 Splunk On-Call
+ *
+ * @packageDocumentation
+ */
+
 export {
   EntitySplunkOnCallCard,
   splunkOnCallPlugin,