docs: add local development guide

[Raakshass]

Closes #259

Summary

This PR adds a Local Development with SDK Backends guide to help users understand and switch between available training backends.

Content Covered

• Backend comparison table (LocalProcess, Container, Kubernetes)
• When to use each backend - quick prototyping vs distributed training vs production
• Code examples for each backend configuration
• Switching between backends - same trainer code works across all backends
• Common operations (list jobs, view logs, wait, delete)
• Troubleshooting tips for each backend

Changes Based on Feedback

Revised from contributor dev setup to user-facing backends guide per @Fiona-Waters feedback.

Checklist

• Follows existing documentation style (RST format)
• Aligned with kubeflow.org Trainer docs structure
• Includes comparison table as requested in Document Local Development #259

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[github-actions]

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[Fiona-Waters]

@Raakshass Thank you for your interest in contributing. The aim of this issue is to add a guide for customers that would like to use our local and container backends. We would like to show them how to easily switch between available backends, when to use each of them etc. Please re-read the issue description and let me know if you have any questions.

[Raakshass]

Thanks @Fiona-Waters for the clarification! I misunderstood the scope - I'll revise my PR to focus on the user-facing local and container backends guide instead of contributor dev setup.

I'll update it to cover:

• Backend comparison (LocalProcess vs Container vs Kubernetes)
• When to use each backend
• Code examples for switching between backends
• Common operations and troubleshooting

Will push the updated content shortly!

[Raakshass]

@Fiona-Waters I updated the PR content described above. I'm unable to edit the PR title to "docs: add local development with SDK backends guide" - could you please update it? Thanks!