Add dev docs page to document creation of new MDX components

[axe312ger]

Add a page to the website that explains how to:

• write custom components and add it to your project
• how to parse and access asset referenced within the MDX source
• explain how we parse components for docs
  • react-docgen
  • MDX support for all descriptions
  • Uses @example doclet to parse examples
  • List available tokens (randomImageId, ...)