init commit for guidelines

[jhnwu3]

This pull request updates the project documentation to better reflect the current community and clarify contribution guidelines. The most significant changes include updating the "About us" section to represent a broader group of contributors, expanding the list of maintainers and contributors, and introducing clear review criteria for new contributions.

Documentation updates:

• Updated the "About us" section in docs/about.rst to indicate that PyHealth is developed and maintained by a diverse community, rather than a single research team.
• Expanded the list of current maintainers and added sections for major reviewers and PyHealth Research Initiative members in docs/about.rst, making contributor roles more transparent and inclusive.

Contribution guidelines:

• Added a new "Review Criteria" section in docs/how_to_contribute.rst to clearly outline expectations for completeness, healthcare relevance, and package fit for all new contributions.

[EricSchrock]

LGTM!