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

[ElijahAhianyo]

ignore here because the md_page macro will try to read the guides which we dont want

[seqre]

I wonder if GuideCategoryItem could be merged with GuideItem as they seem almost the same. Why did you decide to split them? 🤔

[ElijahAhianyo]

I don't think they serve the same purpose, per se. The previous version of CotApp::new accepted MdPage, which was a public-facing representation of a page. When parsing the guide(cot_site::guides::parse_guides), we convert that to MdPageLink(almost similar signature as MdPage), which is an internal representation of the page that gets rendered in the template.

This change also follows the same concept/design; GuideItem is public-facing, whereas GuideCategoryItem is an internal representation of MdPageLink(s) that get rendered in the template