docs: add some top level module documentation

src/lib.rs

@@ -1,2 +1,4 @@
+//! A TUI Canvas client. Also implements [libcanvas](./libcanvas/index.html), an ergonomic Rust wrapper around
+//! common Canvas API operations.
 pub mod libcanvas;
 pub use reqwest::Url;

src/libcanvas.rs

@@ -1,6 +1,7 @@
+//! An ergonomic Rust wrapper around the Canvas API. Built for Cartographer, but can also be used
+//! in other projects.
 use reqwest::header;
-use reqwest::Url;
-use reqwest::{Client, Error};
+use reqwest::{Client, Error, Url};
 use std::env;
 pub mod courses;