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

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.

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)

[ ] 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)
[x] Documentation (update in the documentation)

[x] I have updated the changelog accordingly (required).
[x] My change requires a change to the documentation.
[ ] I have updated the tests accordingly (required for a bug fix or a new feature).
[x] I have updated the documentation accordingly.
[x] I have reformatted the code and checked that formatting passes (required).
[x] I have have fixed all issues found by flake8 (required).
[x] 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).
[x] I have checked that the documentation builds (required).

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