Fix Running on Device link in CUDA backend overview

[pratikgx]

Summary

The "Running on Device" link in the CUDA backend overview points to getting-started.md#running-on-device, which resolves to docs/source/backends/cuda/getting-started.md. That file doesn't exist, so the link is broken. The getting started guide is at docs/source/getting-started.md, so the link is now ../../getting-started.md#running-on-device. This matches the same link in backends/coreml/coreml-overview.md.

Test plan

Docs only. docs/source/getting-started.md exists on main and has a ## Running on Device heading, so the anchor resolves. No docs build was run.

cc @nil-is-all

[pytorch-bot]

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As of commit fe33130 with merge base 0b3d26d ():

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[linux-foundation-easycla]

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[pratikgx]

@pytorchbot label "release notes: none"