Add readthedocs config

[janbridley]

Description

Readthedocs support has been added. I set fail on warning to True, so we need to be sure that the docs build without warning. I will set up CI to check this and assert we do not merge PRs that break the docs.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality and should be merged into the breaking branch

Checklist:

• I am familiar with the Development Guidelines
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• I have updated the changelog and added my name to the credits.