docs: initial content for Subclassing index (#26)

Co-authored-by: Nathan Walker <[email protected]>

Author: Ombuweb

content/guide/subclassing/index.md

@@ -0,0 +1,13 @@
+---
+title: Extending native classes and implementing interfaces in NativeScript
+---
+
+In NativeScript, you can extend native classes and implement interfaces for Android, and conform to iOS protocols. 
+
+See the following articles for examples:
+- [Extending Kotlin/Java classes in NativeScript](/guide/subclassing/extending-classes-and-implementing-interfaces-android#extending-javakotlin-classes-in-nativescript)
+- [Implementing Kotlin/Java interfaces in NativeScript](/guide/subclassing/extending-classes-and-implementing-interfaces-android#implementing-java-kotlin-interfaces-in-nativescript)
+- [Extending Objective-C/Swift classes in NativeScript](/guide/subclassing/extending-classes-and-conforming-protocols-ios.md#extending-ios-classes-in-nativescript)
+- [Conforming to Objective-C/Swift protocols in NativeScript](/guide/subclassing/extending-classes-and-conforming-protocols-ios.md#conforming-to-objective-c-swift-protocols-in-nativescript)
+
+

content/sidebar.ts

@@ -228,6 +228,10 @@ export default [
         text: 'Data Binding',
         link: '/guide/data-binding',
       },
+      {
+        text: 'Extending Native Classes',
+        link: '/guide/subclassing'
+      },
       {
         text: 'Multithreading',
         link: '/guide/multithreading',