[docs] Model API (#3562)
* add modelmixin and unets
* remove old model page
* minor fixes
* fix unet2dcondition
* add vqmodel and autoencoderkl
* add rest of models
* fix autoencoderkl path
* fix toctree
* fix toctree again
* apply feedback
* apply feedback
* fix copies
* fix controlnet copy
* fix copies
