Add Gatsby example

[pomber]

There's a release candidate for MDX v2 support.

Docs: https://github.com/gatsbyjs/gatsby/blob/feat/mdx-v2/packages/gatsby-plugin-mdx/README.md.

[pomber]

I have a working example here: https://github.com/pomber/gatsby-code-hike-example

I'll wait until Gatsby's stable version to add it to the codehike repo.

[github-actions]

🚀 Issue was released in v0.7.4 🚀