packages,plugins: add package documentation

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

packages/dev-utils/api-report.md

@@ -51,6 +51,4 @@ export const EntityGridItem: ({
 }: Omit<GridProps<'div', {}>, 'container' | 'item'> & {
   entity: Entity;
 }) => JSX.Element;
-
-// (No @packageDocumentation comment for this package)
 ```

packages/dev-utils/src/index.ts

@@ -14,5 +14,11 @@
  * limitations under the License.
  */
 
+/**
+ * Utilities for developing Backstage plugins.
+ *
+ * @packageDocumentation
+ */
+
 export * from './components';
 export * from './devApp';