Add ENGR 100 class to user showcase

[EricAndrechek]

ENGR 100 class section 980 - Rocket Science now uses primer spec on its own website.

I am also working on creating the labs and site for section 950 and will turn engr100.org into a landing page once completed. The hope is that other ENGR 100 sections can use our github org to add their own sections to our website.

[seshrs]

Hi @EricAndrechek! 👋

Thanks for using Primer Spec! It’s cool that the Engineering 100 may be transitioning to websites under a single domain.

If you run into issues, have questions, or wish to propose feature requests, feel free to reach out any time on GitHub or email :)