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docs: enhance theme configuration and update local execution guides

- Updated color variables in `conf.py` for improved light and dark mode support.
- Refined CSS styles to utilize new color variables, enhancing visual consistency across the documentation.
- Added architecture diagrams for Docker and Podman backends in their respective guides, clarifying the orchestration process.
- Improved the structure and clarity of the local execution documentation, including error handling and configuration examples.

These changes aim to enhance the overall user experience and visual appeal of the Kubeflow Trainer documentation.

Signed-off-by: Sridhar1030 <sridharpillai75@gmail.com>