Cats-effect: Improve "Concurrency Basics" documentation

[Avasil]

287 is merged and published here

I don't want to stop there, I'm happy to improve and expand it incrementally but it needs a place to gather more feedback. If there is a lot of it we could even make a todo list and complete it together.

@rossabaker has mentioned that he already has few ideas. :)

todo-list

• [ ] continuations
• [ ] stack safety

[Zelenya]

Would some pics be useful?

[alexandru]

Ha, if you guys figure something out, it would be awesome

[Avasil]

Of course @Zelenya !
Also, if you have an idea for a pic but it's not described yet than let me know and I can change it. :D