Write renderer docs and fix broken links

src/renderer.rs

@@ -1,6 +1,33 @@
+//! Write your own renderer!
+//!
+//! There is not a common entrypoint or trait for a __renderer__ in Iced.
+//! Instead, every [`Widget`] constrains its generic `Renderer` type as
+//! necessary.
+//!
+//! This approach is flexible and composable. For instance, the
+//! [`Text`] widget only needs a [`text::Renderer`] while a [`Checkbox`] widget
+//! needs both a [`text::Renderer`] and a [`checkbox::Renderer`], reusing logic.
+//!
+//! In the end, a __renderer__ satisfying all the constraints is
+//! needed to build a [`UserInterface`].
+//!
+//! [`Widget`]: ../widget/trait.Widget.html
+//! [`UserInterface`]: ../struct.UserInterface.html
+//! [`Text`]: ../widget/text/struct.Text.html
+//! [`text::Renderer`]: ../widget/text/trait.Renderer.html
+//! [`Checkbox`]: ../widget/checkbox/struct.Checkbox.html
+//! [`checkbox::Renderer`]: ../widget/checkbox/trait.Renderer.html
 use crate::Layout;
 
-pub trait Renderer {
+/// A renderer able to graphically explain a [`Layout`].
+///
+/// [`Layout`]: ../struct.Layout.html
+pub trait Debugger {
+    /// The color type that will be used to configure the _explanation_.
+    ///
+    /// This is the type that will be asked in [`Element::explain`].
+    ///
+    /// [`Element::explain`]: ../struct.Element.html#method.explain
     type Color: Copy;
 
     /// Explains the [`Layout`] of an [`Element`] for debugging purposes.