Update Customization documentation on how to download gated models.

[Andergraw]

The docs about customization are awesome. It would be amazing to have more info on how to download gated
models (from Hugging Face or CivitAI) without exposing the keys.
With Hugging Face, probably using theur CLI and with CivitAI, just storing the secret securely and adding it as an env variable or something.
Thanks!