Upgrade @mdx-js/mdx to the latest version

[sergiodxa]

Is your feature request related to a problem? Please describe.
Right now we are using an old version of @mdx-js/mdx and upgrading to the latest one breaks the build of the website.

Describe the solution you'd like
Upgrade to the latest version and update the code according to make it work.

Describe alternatives you've considered
Maybe ditch MDX and use normal Markdown without React, this will probably speed up the build time but we need to see how to inline only the required CSS as emotion does.

Maybe use Emotion or another CSS in JS tool to generate the class names only could be an option.