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/11_clone.md

@@ -106,6 +106,3 @@ The compiler implements `Clone` for `MyType` as you would expect: it clones each
 then constructs a new `MyType` instance using the cloned fields.\
 Remember that you can use `cargo expand` (or your IDE) to explore the code generated by `derive` macros.
 
-## References
-
-- The exercise for this section is located in `exercises/04_traits/11_clone`