docs(core): mark TestBed as stable api and add preliminary docs (#11767)

TestBed was accidentaly ommited from the 'stable' api list during the API sweep before final. We do consider it to be stable.

modules/@angular/core/testing/test_bed.ts

@@ -47,7 +47,13 @@ export type TestModuleMetadata = {
 };
 
 /**
- * @experimental
+ * @whatItDoes Configures and initializes environment for unit testing and provides methods for
+ * creating components and services in unit tests.
+ * @description
+ *
+ * TestBed is the primary api for writing unit tests for Angular applications and libraries.
+ *
+ * @stable
  */
 export class TestBed implements Injector {
   /**