The ReadtheDocs build is currently failing because it is unable
to find sphinx_copybutton. This occurs because ReadtheDocs by
default will only install dependencies of the default requirements.txt
at the root.

The fix needs to install the sphinx_copybutton package found in
doc/requirements.txt. ReadtheDocsv2 now provides a better configuration
using a YAML file [1] instead of just the UI and its limited options.

This change adds the new .readthedocs.yaml file that allows us to
control what dependencies are installed via two requirements.txt files,
one in root and one in doc/.

[1] https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

Signed-off-by: Eric Brown <eric_wade_brown@yahoo.com>