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/05_ticket_v2/10_packages.md

@@ -62,6 +62,3 @@ binary crate inside. If you want to create a library crate instead, you can use
 cargo new my-library --lib
 ```
 
-## References
-
-- The exercise for this section is located in `exercises/05_ticket_v2/10_packages`