What's the simplest way to set up a single static webpage that you can edit in Markdown and then converts to HTML? Want to minimize boilerplate templating code and other complexity.

Basically this, but not having to edit raw HTML.

What's the simplest way to set up a single static webpage that you can edit in Markdown and then converts to HTML? Want to minimize boilerplate templating code and other complexity.

Basically this, but not having to edit raw HTML. 

https://motherfuckingwebsite.com

The closest thing to this is GitHub Pages with Jekyll but there's a lot of overhead.

Could even be a CLI that converts Markdown to single HTML file and then uploads to server or whatever deploy workflow on Netlify?

You can reduce some of the overhead by using the web interface on GitHub. 
Add File > Create New File > you're practically in a text/md editor.