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/03_ticket_v1/03_modules.md

@@ -113,6 +113,3 @@ Nonetheless, it can be useful in some cases, like when writing unit tests. You m
 that most of our test modules start with a `use super::*;` statement to bring all the items from the parent module
 (the one being tested) into scope.
 
-## References
-
-- The exercise for this section is located in `exercises/03_ticket_v1/03_modules`