Add documentation (#287)

* Add crate documentation

* Use full path intead of relative path

* Add documentation to core/document/src/color.rs

* Add modification by

* Add @Keavon's modification

* @Keavon's changes

* remove useless let

Co-authored-by: TrueDoctor <dennis@kobert.dev>

core/document/src/lib.rs

@@ -1,3 +1,9 @@
+//! Graphite Document Core Library: `/core/document/`  
+//! A stateless library for updating Graphite design document (GDD) files.
+//! The official Graphite CLI and Editor Core Library are the primary users,
+//! but this library is intended to be useful to any application that wants to link the library for the purpose of updating GDD files by sending edit operations.
+//! Optionally depends on the Renderer Core Library if rendering is required.
+
 pub mod color;
 pub mod document;
 pub mod intersection;