[Examples] Add madebyollin VAE to SDXL LoRA example, along with an explanation

[mnslarcher]

What does this PR do?

This PR adds the madebyollin VAE, along with an explanation of why it's a good idea to replace the original VAE with it in the SDXL LoRA example, as discussed in #4736.

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@sayakpaul

[sayakpaul]

Very clear. Thanks for improving the docs!

[sayakpaul]

Since it's the README of an example, it should not affect the CI. Merging 🚀

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.