Update/docs

[jhnwu3]

This pull request significantly enhances the PyHealth documentation by adding comprehensive "Getting Started" sections, detailed contribution checklists, and best practices for writing fast, reproducible tests. These changes make it much easier for new users and contributors to understand how to use PyHealth's features, access data, and contribute effectively. The documentation now includes step-by-step guides, example workflows, and explicit instructions for using synthetic data in tests, improving both usability and maintainability.

Documentation improvements for onboarding and usage:

• Added "Getting Started" sections to the API documentation for data, datasets, tasks, calib, and interpret, each with links to tutorials, videos, and example workflows illustrating core concepts and usage patterns. [1] [2] [3] [4] [5]

Contributor guidelines and best practices:

• Expanded the contribution guide with a "Data Access for Testing" section, providing resources and instructions for accessing demo datasets and emphasizing the importance of using synthetic data for development and testing.
• Added detailed file checklists and testing requirements for contributing new datasets, tasks, and models, including examples of minimal synthetic data and expected test structures.

Testing and CI/CD improvements:

• Overhauled the "Writing Fast and Performant Tests" section to mandate the use of tiny synthetic data, provide concrete examples, and list explicit do's and don'ts for test speed and reproducibility.

These updates collectively improve the onboarding experience for new users, clarify contribution requirements, and ensure the codebase remains robust and maintainable.