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docs: update theme configuration and enhance CSS for improved navigation

- Updated the Furo theme options in `conf.py` to refine color variables for light and dark modes, enhancing visual consistency.
- Introduced a top navigation bar in the CSS with links to examples, GitHub, Slack, and the blog, improving accessibility and user experience.
- Expanded the CSS file to include a comprehensive color palette and responsive design adjustments for better usability across devices.

These changes aim to enhance the overall look and feel of the Kubeflow Trainer documentation, making it more user-friendly and visually appealing.

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