[docs] Fix broken WebDataset link on “Create a video dataset” page
### What
Fix the "WebDataset documentation" link on the Create a video dataset page to point
to the correct section on the video load guide.

### Why
The link currently points to an external repo, but the Hugging Face docs
have an internal "WebDataset" section under video_load.

### How
- docs/source/video_dataset.mdx: updated link to
  `https://huggingface.co/docs/datasets/main/en/video_load#webdataset`

### Issue
Fixes #7699
