Add keras_cv.models.StableDiffusion() guide

[LukeWood]

This shows how to tune XLA performance, etc.

TODOS:

• run performance benchmarks comparing the huggingface diffusers pytorch implementation to KerasCV's (@ianstenbit)
• write a prompt engineering section
• seek feedback on the prompts

Check out a rendered preview here:
https://nbviewer.org/github/LukeWood/Devoxx/blob/master/Basic%20Generation.ipynb

[LukeWood]

I'd also like to add Divam as an author - waiting to see if he'd like to be included!

[fchollet]

Great stuff! Thanks for the PR!

Would you be ok with me doing a pass over it? We could just merge the .py as-is and keep iterating on it (it won't be displayed until we run add_example)

[LukeWood]

Ok @fchollet feel free to merge and iterate