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/02_basic_calculator/03_if_else.md

@@ -100,6 +100,3 @@ In the example above, each branch of the `if` evaluates to a string literal,
 which is then assigned to the `message` variable.\
 The only requirement is that both `if` branches return the same type.
 
-## References
-
-- The exercise for this section is located in `exercises/02_basic_calculator/03_if_else`