Document horizontal/vertical alignment properties

Right now vertical/horizontal alignment properties are mentioned in the supported CSS properties section, but this is not ideal - we need to document and show examples of using them.

The right place for them would likely be in the various LayoutContainer examples maybe, unless there's a better place (I can't think of one right at the moment).

从受支持的 CSS 属性部分开始，并检查现有的 LayoutContainer 示例。在布局文档中最合适的位置，通过使用示例记录水平方向和垂直方向的对齐属性；当两个属性都得到清晰的解释和演示时，工作即完成。