[INSTALL-9] Transformer Engine uv install should be unified with the main NeMo-RL install

## Problem

The installation guide requires a separate `uv` command to install Transformer Engine with the correct `cudnn` version. It is not clear why this step is separate from the main NeMo-RL install. The split increases friction and the chance of users missing the step.

## Suggested fix

Consolidate TE installation into the main `uv` install flow where possible. If a separate step is unavoidable, add a clear explanation of why and make it visually prominent in the guide.