trainer: Add GPU passthrough documentation for local execution mode

[muzzlol]

Description of Changes

Adds GPU passthrough documentation for Docker and Podman local execution backends:

• docker.md: New "GPU Training" section with prerequisites (NVIDIA Container Toolkit), code example using resources_per_node={"gpu": "1"}, and troubleshooting
• podman.md: New "GPU Training" section with CDI-specific prerequisites, code example, and troubleshooting (including CDI device issues)
• overview.md: Added "NVIDIA GPU Support" row to backend comparison table

Documents platform support (Linux, Windows via WSL2) and notes macOS limitation.

Related Issues

Related: kubeflow/sdk#219

Checklist

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[google-oss-prow]

Hi @muzzlol. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[Arhell]

/ok-to-test

[juliusvonkohout]

@andreyvelich