add: task guide on video classification model fine-tuning. (#20827)

* add: task guide on video classification model fine-tuning.

* apply make style from hf-formatting.

* add: toc entry.

* chore: address PR comments.

Co-authored-by Maria Khalusova

* Reflect Maria's contributions.
Co-authored-by: Maria Khalusova <1065417+MKhalusova@users.noreply.github.com>

* chore: minor correction.

* Apply suggestions from code review
Co-authored-by: Nathan Raw <nxr9266@g.rit.edu>

* PyTorch Video -> PyTorchVideo.

* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* change licensing year.

* minor rewording.

* apply make style.

* address Sylvain's comments.

* replace links.
Co-authored-by: Maria Khalusova <1065417+MKhalusova@users.noreply.github.com>
Co-authored-by: Nathan Raw <nxr9266@g.rit.edu>
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

docs/source/en/_toctree.yml

@@ -73,6 +73,8 @@
       title: Image classification
     - local: tasks/semantic_segmentation
       title: Semantic segmentation
+    - local: tasks/video_classification
+      title: Video classification
     - local: tasks/object_detection
       title: Object detection
     title: Computer Vision