Gatsby: Add two sites to the site showcase

[fk]

Both https://www.edenspiekermann.com/eu/ and https://www.ibm.com/design/ are using Gatsby, and are missing in our site showcase! Let's add them to the end of https://github.com/gatsbyjs/gatsby/blob/master/docs/sites.yml, and tag them as featured: true.

Detailed instructions here: https://www.gatsbyjs.org/contributing/site-showcase-submissions/

[luisFilipePT]

Can I grab this one? (and set myself as assignee)

[fk]

Hey Luis! 👋
Sure, and done! 🙏

[fk]

Thanks again Luis! 🤗