fetch readme.md as mdx

[chenxsan]

There're two types of contents in this repository:

1. those we create right under src/content which we can use mdx safely
2. those read from readme.md file hosted on github, e.g., webpack-contrib/css-loader

This pull request is an attempt to handling the second type as mdx. It might not work as I expected, which means we'll have to maintain both .md and .mdx on this repository.

• close img element
• fix those problems in readme.md files.

Btw, we're leaving those md related configuration at the moment as the downstream repositories might have their own md files, e.g., https://github.com/docschina/webpack.js.org/blob/cn/src/content/plugins/terser-webpack-plugin.md. I'll eliminate those md related configuration in near future.

clean up div[align='center']

Unfortunately the original regular expression for cleaning up div[align='center'] is defective, it can't handle a nested one:

<div align="center">
  <img 
    width="180" 
    height="180" 
    hspace="10"
    alt="PostCSS Logo"
    src="https://api.postcss.org/logo.svg">
  <a href="https://github.com/webpack/webpack">
    <img 
      width="200" 
      height="200" 
      hspace="10"
      src="https://cdn.rawgit.com/webpack/media/e7485eb2/logo/icon.svg">
  </a>
  <div align="center">
    <a href="https://evilmartians.com/?utm_source=postcss">
      <img 
        src="https://evilmartians.com/badges/sponsored-by-evil-martians.svg"
        alt="Sponsored by Evil Martians" 
        width="236" 
        height="54" 
        vspace="10">
    </a>
  </div>
</div>

Thus I'm turning to a remark plugin to resolve it.

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