Later on: change markdown transformer to mdx

[triszt4n]

Problems that this issue would solve:

• More flexibility with using JSX elements in markdowns already
• About page could be entirely mdx based (without that much arbitrary code)
• Fancier posts could be written

Drawbacks:

• Migrate all the plugins under transformer-remark

Gatsby plugin in question: https://www.gatsbyjs.com/plugins/gatsby-plugin-mdx/
Official Gatsby tutorial part: https://www.gatsbyjs.com/docs/tutorial/part-5/