Update data2vec.mdx to include a Colab Notebook link (that shows fine-tuning)
# What does this PR do?

This PR includes a link to a Colab Notebook that shows how to fine-tune the Data2Vec vision model on the task of image classification. 

@sgugger @Rocketknight1 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Yes, sure. Doing it in a while. <|||||>@sgugger done.