Add scrollytelling component to gatsby-theme-blog

[maiertech]

Prior art:

• https://github.com/pomber/gatsby-waves
• https://github.com/code-hike/codehike

Need to figure out whether we can build on Codehike or whether we need to write a component from scratch. The component need to to usable within an MDX file.

[maiertech]

Currently on hold because it's a lot of work.