For your own html create an index.html and no index.md. This HTML file is interpreted through ERB so you can write ruby in it.
Add a CSS folder to your articles folder and all the contents get aromatically copied to the output/static/css folder. The Path of the style.css file gets automatically included in the @CSSblock tag in the base theme. So you need a style.css but in it you can import all other css files if you want to.
Add a js folder to your articles folder and all the files in it get included in the head of the article page. You need to have the @jsBlock in your base theme.
Add all your fonts to the a fonts folder in your articles folder and all get copied to the output/static/fonts folder. So you can include them in your css files.
Add a images folder and link them in your CSS or HTML file.