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/01_intro/00_welcome.md

@@ -88,10 +88,6 @@ Don't move on to the next section until you've solved the exercise for the curre
 
 Enjoy the course!
 
-## References
-
-- The exercise for this section is located in `exercises/01_intro/00_welcome`
-
 ## Author
 
 This course was written by [Luca Palmieri](https://www.lpalmieri.com/), Principal Engineering

book/src/01_intro/01_syntax.md

@@ -114,6 +114,3 @@ tag, the compiler can enforce different rules—e.g. you can't add a string to a
 together.
 If leveraged correctly, types can prevent whole classes of runtime bugs.
 
-## References
-
-- The exercise for this section is located in `exercises/01_intro/01_syntax`