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/renderer/src/lib.rs

@@ -1,3 +1,9 @@
+//! Graphite Renderer Core Library: `/core/renderer/`  
+//! A stateless library (with the help of in-memory and/or on-disk caches for performance) for rendering Graphite's render graph (GRD) files.
+//! The official Graphite CLI and Document 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 rendering Graphite's render graphs.
+//! For example, games can link the library and render procedural textures with customizable parametric input values.
+
 #[cfg(test)]
 mod tests {
 	#[test]