3281 Document how to use pigeon and update to the latest version. (cla: yes, p: video_player, platform-android, platform-ios, waiting for tree to go green)

3490 Remove iOS stubs (cla: yes, p: android_alarm_manager, p: android_intent, p: connectivity, p: espresso, p: file_selector, p: flutter_plugin_android_lifecycle, p: google_maps_flutter, p: google_sign_in, p: image_picker, p: integration_test, p: path_provider, p: shared_preferences, p: url_launcher, p: video_player, platform-ios, platform-linux, platform-macos, platform-web, platform-windows)

3507 [video_player] Update video_player_platform_interface to latest pigeon (cla: yes, p: video_player)