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

Fix all broken links.

book/src/08_futures/07_cancellation.md

@@ -102,7 +102,7 @@ async fn run() {
 
 - Be extremely careful when using `tokio`'s `select!` macro to "race" two different futures.
   Retrying the same task in a loop is dangerous unless you can ensure **cancellation safety**.
-  Check out [`select!`'s documentation](https://docs.rs/tokio/macro.select.html) for more details.  
+  Check out [`select!`'s documentation](https://tokio.rs/tokio/tutorial/select) for more details.  
   If you need to interleave two asynchronous streams of data (e.g. a socket and a channel), prefer using 
   [`StreamExt::merge`](https://docs.rs/tokio-stream/latest/tokio_stream/trait.StreamExt.html#method.merge) instead. 
 - Rather than "abrupt" cancellation, it can be preferable to rely