Resolve documentation build failures for readthedocs ucam-raven

As a developer, I would like to resolve the recent readthedocs documentation build failures 
to ensure our documentation is up-to-date and accessible.

Background

The ucam-raven project documentation builds have been failing recently on the readthedocs:

1. https://readthedocs.org/projects/ucam-raven/builds/
2. https://readthedocs.org/projects/ucam-raven/builds/23712272/
3. https://readthedocs.org/api/v2/build/23712272.txt

Acceptance Criteria

• Investigate and fix the root cause of the failed documentation builds.
• Test the documentation build process.