Markdown is a simple text syntax that can be converted to web documents by many tools.
It is popularized by GitHub.
One big advantage of text format for editing documents is ability to use "diff" tools to trace history of changes in source controls systems.
Markdown editing with Visual Studio Code
Open any Markdown file and press Ctrl+Shift+V in the editor to toggle between the code and a preview of the Markdown file. You can view the preview side-by-side with the file you are editing and see changes reflected in real-time as you edit.
Markdown Basics - User Documentation
GitHub Flavored Markdown - User Documentation
Daring Fireball: Markdown Syntax Documentation