Add example images to widget docs

src/widget/slider.rs

@@ -38,6 +38,8 @@ use crate::{
 ///
 /// Slider::new(state, 0.0..=100.0, value, Message::SliderChanged);
 /// ```
+///
+/// ![Slider drawn by Coffee's renderer](https://github.com/hecrj/coffee/blob/bda9818f823dfcb8a7ad0ff4940b4d4b387b5208/images/ui/slider.png?raw=true)
 pub struct Slider<'a, Message> {
     state: &'a mut State,
     range: RangeInclusive<f32>,