[Docs]: enhance docs of using slot to customize theme

[Timeless0911]

What problem does this feature solve?

https://rspress.dev/guide/advanced/custom-theme#2-use-slot

The doc of using slot to customize theme only provided a demo with <Layout> component, it contains <HomeLayout>, DocLayout, Nav, etc...

  const {
    top,
    bottom,
    beforeDocFooter,
    afterDocFooter,
    beforeDoc,
    afterDoc,
    beforeDocContent,
    afterDocContent,
    beforeSidebar,
    afterSidebar,
    beforeOutline,
    afterOutline,
    beforeNavTitle,
    afterNavTitle,
    beforeNav,
    beforeHero,
    afterHero,
    beforeFeatures,
    afterFeatures,
    afterNavMenu,
  } = props;

We can both set them in <Layout>, but also set them separately in <HomeLayout>, we should add some demo in https://rspress.dev/guide/default-theme/home-page to link something from https://rspress.dev/guide/advanced/custom-theme#2-use-slot.