Add CI job to verify that we have no broken links. (#50)

Fix all broken links.

book/src/03_ticket_v1/05_encapsulation.md

@@ -38,7 +38,7 @@ let ticket = Ticket {
 You've seen this in action in the previous exercise on visibility.  
 We now need to provide one or more public **constructors**—i.e. static methods or functions that can be used
 from outside the module to create a new instance of the struct.  
-Luckily enough we already have one: `Ticket::new`, as implemented in [a previous exercise](02_validation).
+Luckily enough we already have one: `Ticket::new`, as implemented in [a previous exercise](02_validation.md).
 
 ## Accessor methods
 
@@ -60,4 +60,4 @@ You have to write them yourself—they're just regular methods.
 
 - The exercise for this section is located in `exercises/03_ticket_v1/05_encapsulation`
 
-[^newtype]: Or refine their type, a technique we'll explore [later on](../05_ticket_v2/15_outro).
+[^newtype]: Or refine their type, a technique we'll explore [later on](../05_ticket_v2/15_outro.md).

book/src/03_ticket_v1/10_references_in_memory.md

@@ -49,6 +49,6 @@ They just point to a memory location, which _may_ be on the heap, but doesn't ha
 
 - The exercise for this section is located in `exercises/03_ticket_v1/10_references_in_memory`
 
-[^fat]: [Later in the course](../04_traits/06_str_slice) we'll talk about **fat pointers**,
+[^fat]: [Later in the course](../04_traits/06_str_slice.md) we'll talk about **fat pointers**,
 i.e. pointers with additional metadata. As the name implies, they are larger than
 the pointers we discussed in this chapter, also known as **thin pointers**.