Fixed CacheProvider demo in the docs (#2678)

* docs: fix cache-provider demo rendering

* docs: fix cache-provider mdx

* docs: add stub implementation of customPlugin

docs/cache-provider.mdx

@@ -11,6 +11,8 @@ import { CacheProvider, jsx, css } from '@emotion/react'
 import createCache from '@emotion/cache'
 import { prefixer } from 'stylis'
 
+const customPlugin = () => {}
+
 const myCache = createCache({
   key: 'my-prefix-key',
   stylisPlugins: [

site/src/components/Playground.js

@@ -31,6 +31,8 @@ export const scope = {
         return require('@emotion/is-prop-valid')
       case 'facepaint':
         return require('facepaint')
+      case 'stylis':
+        return require('stylis')
       default:
         // eslint-disable-next-line no-throw-literal
         throw `Module "${moduleName}" not found`