Add Support for Subcategories in Guide

[ElijahAhianyo]

With this you should be able to add nested subcategories like this:

vec![
    GuideItem::Page(md_page!("v0.2", "introduction")),
    GuideItem::SubCategory {
      title: "Database",
      pages: vec![
         md_page!("databases/overview"),
         md_page!("databases/queries"),
      ],
   },
],

Refer to the sidebar in this image(cot-rs/cot#515 (comment)) on how this renders

[ElijahAhianyo]

Depends on #82