Copilot BYOK docs polish

[isidorn]

@pierceboggan wrote a nice doc about BYOK

https://docs.google.com/document/d/1An4xz6CQXuN9Z9zPIbTpQnhCBM5uOuU_noEMplxfZsQ/edit?tab=t.0

When we find time we should review this one, and see if our existing BYOK docs could be improved by taking some content out of that

And some good points raised by @lramos15 that we might document (or have it as part of FAQ)

BYOK / BYOM is meant to allow users to tinker with the newest models the moment they are announced all within Copilot Chat. This is why we are just targeting Copilot Pro / Copilot Free.

Limitations for enterprises include

• Embeddings still going through CAPI
• Repository indexing is still going through CAPI
• Query refinement / intent detection / side requests going through CAPI
• Key management / key distribution isn't really figured out