Docs: Add custom fine-tuning tutorial to TrOCR model page

[Ashutosh-4485]

Docs: add community fine‑tuning notebook link to TrOCR page

What does this PR do?

Adds a link to a community tutorial notebook in the TrOCR model docs.

The notebook demonstrates fine-tuning TrOCR on a custom custom dataset and includes multiple training strategies:

• Train all parameters
• Train encoder only (freeze decoder)
• Train decoder only (freeze encoder)
• Train only the last N layers of both encoder and decoder

Tutorial link: https://github.com/Ashutosh-4485/trocr-custom-fine-tune/blob/66010bcdaf45313b829fa83b3c1ba20002e3c323/FineTune_TrOCR_on_Custom_data.ipynb

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

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Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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[stevhliu]

Thanks for adding a tutorial!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.