Docs/Add conversion code to the musicgen docs

[yoinked-h]

What does this PR do?

Adds conversion code to the musicgen docs (the conversion code is quite hidden and needs some changes to be run on other model dirs)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@sanchit-gandhi

[ArthurZucker]

Hey! Updated the people you pinged, let's minimize it 😉

[ArthurZucker]

Thanks 😉

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.