Markdown, AsciiDoc and reStructuredText are markup languages that are used for docs-as-code Documentation.
Markdown
Markdown is used by the majority of Static Site Generators.Because the Markdown syntax is very simple, there is often the need to extend its capabilities by combining Markdown with embedded HTML or even syntax extension, e.g., Python Markdown. There are also different sytax flavors.
ReStructuredText
reStructuredText is widely used in the Python community, e.g., WriteTheDocs and Sphynx. reStructuredText offers more advanced formatting than Markdown and also targets web applications.
AsciiDoc
AsciiDoc is used by some static site generators (e.g., Antora) and is best suited for printing. AsciiDoc offers flexibility without the use of HTML or flavors for essential syntax such as tables, description lists, admonitions (tips, notes, warnings, etc.) and table of contents.