Added KaTeX support and made some changes to the markdown-guide.md

Add ability to import custom CSS files without overwriting templates

This provides the ability to override CSS theme with the config file,
without overriding the template or static files, as digitalcraftsman's
answer in this discourse thread:

https://discourse.gohugo.io/t/how-to-override-css-classes-with-hugo/3033/4

See digitalcraftsman's answer in this discourse thread:
https://discourse.gohugo.io/t/how-to-override-css-classes-with-hugo/3033/4

Lowercase link destination for categories and tags

README: `scaffold` to `exampleSite`, see 811e0c60