Add learn track for C

[dawnandrew100]

Currently, the C track links out to some resources to learn outside of Exercism. However, I don't believe that https://www.learn-c.org/ is actively maintained and other languages attached to this site are extremely outdated (such as using the old formatting style for strings in python instead of f-strings). And while I'm sure CS50X is a fantastic resource, it appears that it focuses on a lot more than just the C language. This isn't good news for people who are interested in learning specifically about C and not interested in spending 12 weeks learning about all of computer science.

It would be fantastic to have a learn track on Exercism for C instead of having to rely on the good graces of outside individuals.