[Feature Request] Add mentoring CTA to Exercism download command

[iHiD]

I was inspired by this

Once we've got the mentoring signup integrated into the site, we could add something like:

Exercism relies on volunteer mentoring. If you would like to be a mentor, please sign up at:
https://exercism.io/mentor

[kytrinyx]

This is a great idea!

[kytrinyx]

@PSalant726 took a stab at this one in #749 which started a really great conversation about trade-offs and user experience and implementation approaches.

The TL;DR is that @iHiD and I will work our way through the implementation and UX questions, and then open issues for implementation for both the API and update this issue with implementation notes.