feat(okidoc-site): render react components inside markdown

[bodia-uz]

inspired by https://using-remark.gatsbyjs.org/custom-components/ so has same caveats

Example:

# site.yml
...
mdComponents:
  path: ./docs/playground-components.js
  # NOTE: `externalScripts` and `externalLinks` may affect your scripts and styles
  # externalStyles: []
  # externalScripts: []
...

// playground-components.js

import React from 'react';

export default {
  counter: () => <span>!COUNTER!</span>
};

<!-- index.md -->

# Counter

<counter></counter>

The additional example for playable:
https://github.com/wix/playable/compare/feature/okidoc-demo-components

[kupriyanenko]

Please change externalLinks to externalStyles