Replace mdx asset link resolver / parser with Contentful references

[axe312ger]

The approach of automatically parsing asset ids from MDX is to complicated to maintain. We should replace this technique via Contentful:

• New ContentType "MediaReference". Fields:
  • Asset ID
  • Asset context (to determine how it will be rendered)

Our custom editor / field UI extension could fill the field for the user. Investigation is need how to achieve this properly.

See: https://github.com/axe312ger/gatsby-mdx-suite/blob/master/gatsby-theme-mdx-suite/gatsby-node.js#L22-L54