Merge Android UI Performance into Performance doc, reorder sidebar

Summary:
Doing some cleanup in preparation for CRNA.
Recommend `FlatList` and React Navigation for perf.
Tag docs that may only apply to apps ejected from CRNA. Currently has no effect.
Closes https://github.com/facebook/react-native/pull/12692

Differential Revision: D4654077

Pulled By: hramos

fbshipit-source-id: 1245d80d66e37d9dca9e9daf23e8b93c65cd1bf7

docs/NativeModulesIOS.md

@@ -4,8 +4,9 @@ title: Native Modules
 layout: docs
 category: Guides (iOS)
 permalink: docs/native-modules-ios.html
+banner: ejected
 next: native-components-ios
-previous: gesture-responder-system
+previous: upgrading
 ---
 
 Sometimes an app needs access to platform API, and React Native doesn't have a corresponding module yet. Maybe you want to reuse some existing Objective-C, Swift or C++ code without having to reimplement it in JavaScript, or write some high performance, multi-threaded code such as for image processing, a database, or any number of advanced extensions.
@@ -402,13 +403,13 @@ You will receive a warning if you expend resources unnecessarily by emitting an
 }
 
 // Will be called when this module's first listener is added.
--(void)startObserving { 
+-(void)startObserving {
     hasListeners = YES;
     // Set up any upstream listeners or background tasks as necessary
 }
 
 // Will be called when this module's last listener is removed, or on dealloc.
--(void)stopObserving { 
+-(void)stopObserving {
     hasListeners = NO;
     // Remove upstream listeners, stop unnecessary background tasks
 }