packages,plugins: add package documentation

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

packages/errors/api-report.md

@@ -79,6 +79,4 @@ export function serializeError(
     includeStack?: boolean;
   },
 ): SerializedError;
-
-// (No @packageDocumentation comment for this package)
 ```

packages/errors/src/index.ts

@@ -14,5 +14,11 @@
  * limitations under the License.
  */
 
+/**
+ * Common utilities for error handling within Backstage
+ *
+ * @packageDocumentation
+ */
+
 export * from './errors';
 export * from './serialization';