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_v1/01_struct.md

@@ -137,6 +137,3 @@ let is_open = Ticket::is_open(ticket);
 The function call syntax makes it quite clear that `ticket` is being used as `self`, the first parameter of the method,
 but it's definitely more verbose. Prefer the method call syntax when possible.
 
-## References
-
-- The exercise for this section is located in `exercises/03_ticket_v1/01_struct`