[Train] Update elastic training accelerator examples in docs

[avigyabb]

Description

Updates the Ray Train elastic training docs to show accelerator-specific examples for both GPUs and TPUs.

This removes the outdated TPU limitation and adds a JaxTrainer example for elastic training across TPU slices. The TPU example documents that elastic worker ranges must align with complete TPU slices, matching the existing accelerator guide and ScalingConfig validation behavior.

Screenshots:

[gemini-code-assist]

Code Review

This pull request updates the elastic training documentation to include support for TPUs. It replaces the single GPU example with a tabbed set showing both GPU and TPU configurations, and removes the limitation stating that TPU elastic training is unsupported. The feedback points out a minor typo with double periods in the TPU example description.

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[liulehui]

Thank you!

Could you follow https://docs.ray.io/en/latest/ray-contribute/docs.html this doc to start the doc server locally to view the new changes, plz also attach screenshots of the corresponding doc change part in the PR description.