Fix registration page

[davelosert]

As soon as we'll merge #843 , the registration page will look a bit messed up:

We should fix this before opening the registration.

[lowsky]

After a quick look into it, I found that prism code highlighting does not out-of-the-box together with react.

Prism highlighting needs to be synced with react's rendering lifecycle.

Some details or background:

• https://betterstack.dev/blog/code-highlighting-in-react-using-prismjs/#heading-the-simplest-way-to-use-prismjs-in-react
• https://github.com/FormidableLabs/prism-react-renderer

Maybye, a better alternative will be to use any gatsby markdown plugin...

[lowsky]

-> https://www.gatsbyjs.org/packages/gatsby-plugin-mdx/