Link to the new paper (on arXiv) in the documentation

[jank324]

Description

Links to the new differentiable Cheetah paper on arXiv, rather than the old IPAC paper, which is not up-to-date with Cheetah as it is now.

Motivation and Context

The new paper matches Cheetah today much more closer and should now be considered the main reference.

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

Note: We are using a maximum length of 88 characters per line

[jank324]

Hopefully this is the last fix for #94.