docs(router): make InitialNavigation part of the public API (#32707)

`InitialNavigation` is used in `ExtraOptions`, which is already part of
the public API. Thus, `InitialNavigation` should be too. Not publicly
exporting it from `router/index.ts` seems an omission, since the type is
already annotated with the `@publicApi` JSDoc tag.

By publicly exporting `InitialNavigation`, it will also correctly appear
in the API docs on angular.io.

PR Close #32707

tools/public_api_guard/router/router.d.ts

@@ -148,6 +148,9 @@ export declare class GuardsCheckStart extends RouterEvent {
     toString(): string;
 }
 
+/** @deprecated */
+export declare type InitialNavigation = true | false | 'enabled' | 'disabled' | 'legacy_enabled' | 'legacy_disabled';
+
 export declare type LoadChildren = LoadChildrenCallback | DeprecatedLoadChildren;
 
 export declare type LoadChildrenCallback = () => Type<any> | NgModuleFactory<any> | Observable<Type<any>> | Promise<NgModuleFactory<any> | Type<any> | any>;