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/06_ticket_management/06_lifetimes.md

@@ -77,8 +77,5 @@ You can think of `'_` as a **placeholder** for the lifetime of the `&self` refer
 See the [References](#references) section for a link to the official documentation on lifetime elision.\
 In most cases, you can rely on the compiler telling you when you need to add explicit lifetime annotations.
 
-## References
-
-- [std::vec::Vec::iter](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.iter)
 - [std::slice::Iter](https://doc.rust-lang.org/std/slice/struct.Iter.html)
 - [Lifetime elision rules](https://doc.rust-lang.org/reference/lifetime-elision.html)