Document extensions

Two kinds of extensions as described in https://youtu.be/hNwQPJy-XZY:

• How to make an existing or new primitive component work on more platforms - 17:27 (t=1047 seconds)
• Example of adding a new checkbox extension - 19:17 (t=1156 seconds)

The trick is that for each primitive there would be JavaScript files such as:

• index.js - for web platform (using react-native-web)
• index.android.js - for Android (using react-native)
• index.ios.js - for iOS (using react-native)
• ...

I was not aware of this support for either of the two kinds of extensions until very recently (unfortunately I lost track of which discussion I found https://youtu.be/hNwQPJy-XZY in). I hope we can get this more formally documented in the near future.

P.S. Here are some examples of new "primitive" components (first 2 by myself):

• react-primitives-input
• react-primitives-scrollview
• react-primitives-svg

This description was also updated to make it clear that I meant there are two types of extensions: adding new "primitive" components and supporting additional platforms.

Start by reviewing the linked video at 17:27 and 19:17 and the platform examples listed in the issue: index.js, index.android.js, and index.ios.js. Document both extension types—adding primitive components and supporting additional platforms—with examples such as react-primitives-input, react-primitives-scrollview, and react-primitives-svg; done means the extension process is formally explained.