Automatically add exercise links to sections. (#52)

We use an mdbook preprocessor to automatically generate links to the relevant exercise for each section.
We remove all existing manual links and refactor the deploy process to push the rendered book to a branch.

book/src/04_traits/06_str_slice.md

@@ -115,6 +115,3 @@ If a method returns a `&String`, you're promising that there is heap-allocated U
 If a method returns a `&str`, instead, you have a lot more freedom: you're just saying that _somewhere_ there's a
 bunch of text data and that a subset of it matches what you need, therefore you're returning a reference to it.
 
-## References
-
-- The exercise for this section is located in `exercises/04_traits/06_str_slice`