Beginner-level documentation

[This request on r/haskellquestions](https://www.reddit.com/r/haskellquestions/comments/rg524w/beginner_level_tutorial_bytestring/) indicates that `bytestring` should be more accessible for Haskell beginners.

To improve the situation, I suggest we add:

* Doctest-style (`>>>`) examples on most functions.
- To avoid excessive visual noise, the examples should be in collapsible `==== __Examples__` sections

* A (brief) tutorial, ideally as a [tutorial module](https://github.com/sjakobi/awesome-haskell-tutorial-modules#tutorial-modules).