feat(docs): add local development guide

[sohaa-khan11]

What this PR does / why we need it:
Adds a comprehensive Local Development Guide to the Kubeflow SDK documentation.
The current documentation briefly mentions local backends but does not provide a centralized, practical guide covering:

1. Backend comparison (Local Process, Docker, Podman, Kubernetes)
2. Switching between backends
3. Complete runnable training example
4. Debugging and troubleshooting guidance
5. Runtime configuration details
6. Recommended development workflow

This PR introduces a structured guide to improve developer onboarding and local development clarity.
The documentation was carefully reviewed and validated against the current SDK APIs. All examples were verified manually.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #259

Checklist:

• Docs included if any changes are user facing

[github-actions]

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[google-oss-prow]

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[Copilot]

Pull request overview

Adds a new Local Development Guide to the Kubeflow SDK docs to centralize guidance on running training locally (process/container) and transitioning to Kubernetes, and links it from the main docs navigation and Getting Started page.

Changes:

• Added a new local-development-guide.md page covering backend options, switching, an end-to-end example, and troubleshooting.
• Updated the main docs toctree to include the new guide.
• Added a “Local Development” card to the Getting Started landing page.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/source/local-development-guide.md	Introduces the new local development guide content and examples.
docs/source/index.rst	Adds the new guide into the main docs navigation (toctree).
docs/source/getting-started/index.rst	Adds a “Local Development” navigation card pointing to the new guide.