마크다운 튜토리얼
https://daringfireball.net/projects/markdown/
https://spec.commonmark.org/dingus/
https://johnmacfarlane.net/babelmark2/faq.html
https://www.markdownguide.org
https://dave.autonoma.ca/blog/2019/05/22/typesetting-markdown-part-1/
http://idratherbewriting.com/2013/06/04/exploring-markdown-in-collaborative-authoring-to-publishing-workflows/
https://en.wikipedia.org/wiki/Markdown#Example
https://docs.gitlab.com/ee/user/markdown.html
https://docs.github.com/en/github/writing-on-github/basic-writing-and-formatting-syntax