Address documentation feedback

- Fix broken links
  - Mention generic `Color` in some widgets
  - Clarify meaning of renderer-agnostic in usage/overview example

src/widget/button.rs

@@ -242,11 +242,11 @@ pub enum Class {
 
 /// The renderer of a [`Button`].
 ///
-/// Your [`core::Renderer`] will need to implement this trait before being
+/// Your [renderer] will need to implement this trait before being
 /// able to use a [`Button`] in your user interface.
 ///
 /// [`Button`]: struct.Button.html
-/// [`core::Renderer`]: ../../core/trait.Renderer.html
+/// [renderer]: ../../renderer/index.html
 pub trait Renderer {
     /// Draws a [`Button`].
     ///