Add CustomersDot setup back to 3rd party LLM docs

What does this MR do and why?

After Duo Chat has gone GA, CustomersDot is required to test self-managed AI features locally.

This MR adds them back to the docs.

MR acceptance checklist

✅

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

N/A

How to set up and validate locally

N/A