One of the various markup languages that has probably gained the most traction.
Alternatives are Asciidoc (adoc) and reStructureText (rST). Also, Markdown has a few different flavors such as Github-Flavored Markdown (GFM). Dewan Ahmed has a great article covering this called Markdown, Asciidoc, or reStructuredText - A Tale of Docs-as-Code.
Mikey Lombardi has an excellent article that walks you through how to write down markdown. Mikey is one of the amazing Documentarians at Microsoft working on PowerShell docs. His article Writing Markdown is geared towards writing in VSCode, but it has some phenomenal recommendations.