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.

tools/public_api_guard/core/testing/index.d.ts

@@ -56,7 +56,7 @@ export declare type MetadataOverride<T> = {
 /** @experimental */
 export declare function resetFakeAsyncZone(): void;
 
-/** @experimental */
+/** @stable */
 export declare class TestBed implements Injector {
     ngModule: Type<any>;
     platform: PlatformRef;