Use furo theme for documentation

[tommy-waltmann]

Description

This PR changes the documentation theme from the sphinx readthedocs theme to the furo theme.

Motivation and Context

The furo theme is more customizable, has better search display, different lighting modes, and overall looks a bit cleaner than the the readthedocs theme.

How Has This Been Tested?

readthedocs will build and render the documentation changes in the new theme by default.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.