RTD seems to have moved their documentation to https://docs.readthedocs.io/en/latest/. RTD's documentation specific to MkDocs is at https://docs.readthedocs.io/en/latest/intro/getting-started-with-mkdocs.html, however, that doesn't offer any instruction on how to deploy your MkDocs based site.
As it happens, the process for deploying to RTD is the same regardless of which of the supported static site generators you use (Sphinx or MkDocs) and is documented at https://docs.readthedocs.io/en/latest/intro/import-guide.html:
To import a public documentation repository, visit your Read the Docs
dashboard and click Import. For private repositories, use the Read
the Docs commercial solution.
If you have connected your Read the Docs account to GitHub,
Bitbucket, or GitLab, you will see a list of your repositories that we
are able to import. To import one of these projects, just click
the import icon next to the repository you’d like to import. This will
bring up a form that is already filled with your project’s
information. Feel free to edit any of these properties, and then click
Next to build your documentation.
Note that if you do not have (or want to have) your GitHub account connected with your RTD account, then you will need to manually import your repo, which is documented here. The documentation continues:
Within a few seconds of completing the import process, your code will
automatically be fetched from your public repository, and the
documentation will be built. Check out our Build Process page to
learn more about how Read the Docs builds your docs, and to
troubleshoot any issues that arise.
Some documentation projects require additional configuration to build
such as specifying a certain version of Python or installing
additional dependencies. You can configure these settings in a
readthedocs.yml
file. See our Configuration File docs for more
details.
Last I checked, RTD was not using the most recent release of MkDocs by default, so you may want to make use of the readthedocs.yml
file to specify the MkDocs version you have written your docs against as well as any third party plugins, themes, and/or Markdown Extensions you are using.