Focus NIM guide on the inference API (#1135)

> Refocus the NIM Inference guide on the HTTP API and the role of
published NIMs in enterprise inference.
>
> Replace the package tutorial with an overview of deployment,
prediction requests, context reuse, and application and operator
responsibilities.
>
> Change only `docs/source/nim.md`.

---------

Signed-off-by: Manush Murali <manushm@nvidia.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: Manushpm8