Document warp-frontend play commands in the quick start

[hujc7]

Description

The Newton warp-environments quick start listed only train commands, so there was no
documented way to run inference on a policy trained with --frontend warp.

Split the quick start into Training and Playing a Trained Policy, so every train
example has a matching play example in both the uv and isaaclab.sh tabs, and note
that --video is rejected on the warp path.

Fixes nvbug 6531264.

Page: https://isaac-sim.github.io/IsaacLab/develop/source/overview/core-concepts/physical-backends/newton/warp-environments.html#quick-start

Type of change

• Documentation update

Validation

Every newly documented command was run end-to-end on an RTX A6000 (5 iterations, small num_envs):

Command	Result
train --task Isaac-Cartpole-Direct --frontend warp presets=newton_mjwarp	✅ Training time: 2.78 seconds
train --task Isaac-Velocity-Flat-AnymalD --frontend warp presets=newton_mjwarp	✅ Training time: 5.54 seconds
play --task Isaac-Cartpole-Direct --frontend warp ... --checkpoint latest --visualizer newton	✅ loaded model_4.pt, stepped under InteractiveSceneWarp
play --task Isaac-Velocity-Flat-AnymalD --frontend warp ... --checkpoint latest --visualizer newton	✅ loaded model_4.pt, stepped under InteractiveSceneWarp
play ... --frontend warp --video	✅ rejected with ValueError: --video is not supported with --frontend 'warp'

No changelog fragment: the change touches only docs/, and tools/changelog/cli.py check develop passes.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added my name to the CONTRIBUTORS.md or my name already exists there