adding document mode

I've been thinking a lot about the treehouse and I feel like it's time to say goodbye to the tree format.

Justfile

@@ -1,7 +1,7 @@
 port := "8080"
 
 serve:
-    cargo watch -- cargo run -- serve --port {{port}}
+    RUST_BACKTRACE=1 cargo watch -- cargo run -- serve --port {{port}}
 
 fix:
     cargo run -- fix-all --apply