add: task guide on video classification model fine-tuning.
This PR adds a task guide on fine-tuning video classification models. 
@nateraw I leveraged the video classification pipeline with the custom fine-tuned model and a URL from the [UCF-101 subset](https://huggingface.co/datasets/sayakpaul/ucf101-subset) (`avi` format). It worked like a charm! 🔥<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@MKhalusova pushed a few changes. Will add another commit adding you as a co-author when you're back online. <|||||>@amyeroberts thank you! Addressed all your comments. <|||||>@sgugger one small pending part is https://github.com/huggingface/transformers/pull/20827#discussion_r1061285987. Once it's resolved, we should be good to merge. <|||||>Updated the links. Will wait for the tests to pass and will merge afterward. 