Mkdocs: default to "docs" for docs_dir (#7766)
This value has been the default since the start of the project
https://github.com/mkdocs/mkdocs/blob/0.13.0/mkdocs/config/defaults.py#L44
We should respect that default value,
(we should actually don't edit it at all https://github.com/readthedocs/readthedocs.org/issues/2483).
So, instead of trying to guess the docs dir always,
only try to guess it if the user doesn't have mkdocs.yaml file.
This should avoid any backwards compatibility and avoid weird bugs to
new users.
Closes https://github.com/readthedocs/readthedocs.org/issues/7539
