[docs] add doc for PixArtSigmaPipeline

[lawrence-cj]

What does this PR do?

This PR adds docs for PixArtSigmaPipeline. Cc: @sayakpaul @yiyixuxu

[yiyixuxu]

for the failing test, can you add the new doc page here?

https://github.com/lawrence-cj/diffusers/blob/d5f4c217b5f6ac4fd23b4ee627611e0fbe0707a7/docs/source/en/_toctree.yml#L308

[lawrence-cj]

thanks! also a note, can we change the type for scheduler to KarrasDiffusionSchedulers? this here indicates that sigma pipeline only works with DPM https://github.com/lawrence-cj/diffusers/blob/d5f4c217b5f6ac4fd23b4ee627611e0fbe0707a7/src/diffusers/pipelines/pixart_alpha/pipeline_pixart_sigma.py#L191C20-L191C47

Cool. Changed.

[lawrence-cj]

for the failing test, can you add the new doc page here?

https://github.com/lawrence-cj/diffusers/blob/d5f4c217b5f6ac4fd23b4ee627611e0fbe0707a7/docs/source/en/_toctree.yml#L308

Fixed all the request here. Cc: @sayakpaul @yiyixuxu

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[lawrence-cj]

Gentle ping here. @yiyixuxu