packages,plugins: add package documentation

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

packages/backend-common/api-report.md

@@ -574,6 +574,4 @@ export function useHotCleanup(
 
 // @public
 export function useHotMemoize<T>(_module: NodeModule, valueFactory: () => T): T;
-
-// (No @packageDocumentation comment for this package)
 ```

packages/backend-common/src/index.ts

@@ -14,6 +14,12 @@
  * limitations under the License.
  */
 
+/**
+ * Common functionality library for Backstage backends
+ *
+ * @packageDocumentation
+ */
+
 export * from './cache';
 export * from './config';
 export * from './database';