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_v2/04_if_let.md

@@ -65,6 +65,3 @@ Both `if let` and `let/else` are idiomatic Rust constructs.\
 Use them as you see fit to improve the readability of your code,
 but don't overdo it: `match` is always there when you need it.
 
-## References
-
-- The exercise for this section is located in `exercises/05_ticket_v2/04_if_let`