Replace Playground with JSX Editor (#1500)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1500

Inspired by the frequent usage of Expo snacks to run RN, to repro Yoga issues, this replaces the Playground port with a new ground up Playground UI. This UI right now is pretty simple, with a JSX editor which creates a Yoga tree, which is then rendered using the WebAssembly variant of Yoga.

There are a lot of ways we can continue to improve this, but this merges the foundation. Subjectively, I find this more useful as a tool to play with Yoga behavior than the GUI.

This also replaces some of the bits of the homepage, and adds a playground entrypoint (though it's pretty identical to the one I've been testing on the home page).

Reviewed By: yungsters

Differential Revision: D51963201

fbshipit-source-id: 1265cb1784151b685686e189d47ecd42cbacdf8f

website-next/sidebars.cjs

@@ -22,19 +22,6 @@
 const sidebars = {
   // By default, Docusaurus generates a sidebar from the docs folder structure
   docsSidebar: [{type: 'autogenerated', dirName: '.'}],
-
-  // But you can create a sidebar manually
-  /*
-  tutorialSidebar: [
-    'intro',
-    'hello',
-    {
-      type: 'category',
-      label: 'Tutorial',
-      items: ['tutorial-basics/create-a-document'],
-    },
-  ],
-   */
 };
 
 module.exports = sidebars;