Welcome to the Read the Docs Documentation!
Read The Docs primarily uses python, sphinx, and ReStructured Text (RST) to create robust and aesthetically pleasing documentation.
MkDocs uses python and Markdown but MkDocs also has a Read the Docs theme. While the sphinx based version can use markdown as well, we will focus on the MkDocs version as it's more tailored to markdown.
There are some advantages to RST (eg PDF and LAtex generation among others) Read this post for some discussion on using RST vs Markdown. In my opinion the general consensus is that RST is more robust for technical documentation whereas markdown was created with simplicity in mind and as a result can have some limitations.