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

Fix all broken links.

book/src/02_basic_calculator/04_panics.md

@@ -1,6 +1,6 @@
 # Panics
 
-Let's go back to the `speed` function you wrote for the ["Variables" section](02_variables).
+Let's go back to the `speed` function you wrote for the ["Variables" section](02_variables.md).
 It probably looked something like this:
 
 ```rust
@@ -38,7 +38,7 @@ fn main() {
 }
 ```
 
-There are other mechanisms to work with recoverable errors in Rust, which [we'll cover later](../05_ticket_v2/06_fallibility).
+There are other mechanisms to work with recoverable errors in Rust, which [we'll cover later](../05_ticket_v2/06_fallibility.md).
 For the time being we'll stick with panics as a brutal but simple stopgap solution.
 
 ## References