Fix Readthedocs build

[EwoutH]

The Readthedocs builds are currently failing.

• Last passing build: https://readthedocs.org/projects/platypus/builds/21890213/
• First failing build: https://readthedocs.org/projects/platypus/builds/22247279/

The configuration file required to build documentation is missing from your project. Add a configuration file to your project to make it build successfully. Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html

It seems a .readthedocs.yaml is now required and should be added to this repo.