There are a few different types of Markdown apps. Many of them work like Notational Velocity/nvAlt/Obsidian where it's a folder of plain text files, you type in Markdown primarily, and the editor just applies some formatting after the fact, akin to syntax highlighting. IMO its much nicer than looking at just the Markdown, but it also has the advantage of remaining 100% portable text.
If there is a nice neovim plugin for rendering markdown as you type it, I would love to hear about it.
I have no idea where it came from but my current setup does it nicely.