[docs] AnimateDiff Video-to-Video (#6712)
* add animatediff vid2vid to docs
* Update docs/source/en/api/pipelines/animatediff.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
* apply suggestions from review
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
