Config file for Read The Docs

[wigging]

This adds a configuration file to the docs/ directory for Read The Docs. When setting up the project on the Read The Docs website, the following need to be considered:

• Set the path to the config file to docs/.readthedocs.yaml; otherwise, Read The Docs will look in the root directory of the project for the configuration
• Set latest to the dev branch and stable to the main branch

This resolves #170 to make the documentation viewable online.