Add link to GitHub Skills on Propose a change

[paulosacramento]

Hello, GitHub has released a brand new learning platform today.

I think GitHub Skills could be a very good addition to Propose a change.

Introduction to GitHub and Communicate using Markdown seem specially relevant.

Any ideas?

[sbddesign]

If we want to teach readers how to edit markdown, then the "applying changes" paragraph seems the most relevant. I can envision a sentence like "Most of the Guide's content is formatted in markdown. If you are not familiar with markdown, you can learn more about it here (link to some tutorial). Also, maybe just a cheat sheet for markdown formatting would be easier?

Could also link to the GitHub skills page as a whole somewhere towards the beginning or end of a document. "A complete course in using GitHub is beyond the scope of the Design Guide, but if you are looking to learn more, here is a self-guided tutorial series by GitHub at this link..."

[GBKS]

I created a PR for this with #928. It's a nice addition.