message: doc: added an example and documentation for Spacing
diff: use crate::*;
  +/// Define space between elements.
  +/// More information at <https://developer.mozilla.org/en-US/docs/Web/CSS/gap>.
  #[derive(Default)]
  pub struct Spacing<'a> {
  + /// Static css properties used by zoon.
  static_css_props: StaticCSSProps<'a>,
  }
  impl<'a> Spacing<'a> {
  + /// Add space in pixels between elements. It needs to be set on the
  + /// container.
  + /// More information at <https://developer.mozilla.org/en-US/docs/Web/CSS/gap>.
  + /// # Example
  + /// ```no_run
  + /// use zoon::*;
  + ///
  + /// let grid = Row::new()
  + /// .s(Spacing::new(10))
  + /// .item(Column::new().item("first column"))
  + /// .item(Column::new().item("second column"));
  + /// ```
  pub fn new(spacing: u32) -> Self {
  let mut this = Self::default();
  this.static_css_props.insert("gap", px(spacing));
  