Improve introduction documentation

[rssh]

Track documentation from the point of novice and rewrite parts, which are hard to follow.
// And not forget about experienced developers' paths, who will not want to read 'Monad-101' intro for documentation on each library. Maybe write in addition a separate introduction to the library?

[ScoreUnder]

For me, I first came into this project only knowing the user side of scala-async, which means a "Quick Start" for users like me would hopefully amount to little more than this snippet:

import cps.*
import cps.monads.FutureAsyncMonad