Update native iOS API doc links from objcdoc to ios-embedder

[jmagman]

The iOS API doc paths were changed, but some website links still point to the old format. For example, https://api.flutter.dev/objcdoc/Classes/FlutterMethodChannel.html is now https://api.flutter.dev/ios-embedder/interface_flutter_method_channel.html (note the snake casing change as well)

src/_includes/docs/platform-view-perf.md:[`FlutterTextureRegistry`]: {{site.api}}/objcdoc/Protocols/FlutterTextureRegistry.html
src/ui/assets/assets-and-images.md:[`FlutterPluginRegistrar`]: {{site.api}}/objcdoc/Protocols/FlutterPluginRegistrar.html
src/ui/assets/assets-and-images.md:[`FlutterViewController`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html
src/add-to-app/performance.md:[`FlutterViewController`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html
src/add-to-app/performance.md:[`FlutterViewController initWithProject: nibName: bundle:`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html#/c:objc(cs)FlutterViewController(im)initWithProject:nibName:bundle:
src/add-to-app/performance.md:[`initWithEngine: nibName: bundle:`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html#/c:objc(cs)FlutterViewController(im)initWithEngine:nibName:bundle:
src/add-to-app/performance.md:[ios-engine]: {{site.api}}/objcdoc/Classes/FlutterEngine.html
src/add-to-app/performance.md:[`runWithEntrypoint:`]: {{site.api}}/objcdoc/Classes/FlutterEngine.html#/c:objc(cs)FlutterEngine(im)runWithEntrypoint:
src/add-to-app/ios/add-flutter-screen.md:[`FlutterEngine`]: {{site.api}}/objcdoc/Classes/FlutterEngine.html
src/add-to-app/ios/add-flutter-screen.md:[`FlutterViewController`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html
src/add-to-app/ios/add-flutter-screen.md:[`popRoute()`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html#/c:objc(cs)FlutterViewController(im)popRoute
src/add-to-app/ios/add-flutter-screen.md:[`pushRoute()`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html#/c:objc(cs)FlutterViewController(im)pushRoute:
src/add-to-app/ios/add-flutter-screen.md:[`runWithEntrypoint`]: {{site.api}}/objcdoc/Classes/FlutterEngine.html#/c:objc(cs)FlutterEngine(im)runWithEntrypoint:
src/add-to-app/index.md:[ios-engine]: {{site.api}}/objcdoc/Classes/FlutterEngine.html
src/add-to-app/index.md:[`FlutterViewController`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html
src/add-to-app/multiple-flutters.md:[`FlutterViewController`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html
src/platform-integration/ios/platform-views.md:[`FlutterPlatformView`]: {{site.api}}/objcdoc/Protocols/FlutterPlatformView.html
src/platform-integration/ios/platform-views.md:[`FlutterPlatformViewFactory`]: {{site.api}}/objcdoc/Protocols/FlutterPlatformViewFactory.html
src/platform-integration/platform-channels.md:[`MethodChanneliOS`]: {{site.api}}/objcdoc/Classes/FlutterMethodChannel.html
src/platform-integration/android/restore-state-android.md:[`FlutterViewController`]: {{site.api}}/objcdoc/Classes/FlutterViewController.html

@sfshaza2 do we have any tooling that would have caught these are pointed to 404 links?

cc @gspencergoog flutter/engine#43915