Add docs on how to setup a self-hosted model with a LiteLLM proxy
What does this MR do and why?
Adding a section on how to setup a custom model with LiteLLM.
Follow-up from !153966 (merged). Closes !156502 (merged).
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How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
Edited by Bruno Cardoso