Add/newsletter + update install docs for early release of PyHealth 2.0

[jhnwu3]

This pull request updates the documentation to announce the early full release of PyHealth 2.0, clarify installation instructions, and highlight major new features and improvements. It also updates references to the legacy version and adds a newsletter entry detailing the 2.0 release and future plans.

Key documentation and communication updates:

• PyHealth 2.0 Release Announcement

  • Added a new newsletter entry (docs/newsletter/2026-01-pyhealth-2.0-release.rst) announcing the early release of PyHealth 2.0, summarizing major new features (dynamic memory support, parallelization, new models/datasets, improved interpretability), and outlining the future roadmap for the toolkit. [1] [2] [3]

• Installation Instructions and Versioning

  • Updated installation instructions in both README.rst and docs/install.rst to recommend installing the latest PyHealth 2.0 release with pip install pyhealth, and clarified that this now installs version 2.0 rather than the legacy 1.16 version. The instructions also specify Python version requirements and highlight the improved features in 2.0. [1] [2]
  • Clarified that the legacy version (1.16) is still available for backward compatibility and supports Python 3.9+, but new users are encouraged to use PyHealth 2.0. [1] [2]

• Tutorial and Reference Updates

  • Updated tutorial and video references in README.rst and docs/tutorials.rst to refer to "PyHealth 1.16" instead of "PyHealth 1.6" for consistency with the legacy versioning. [1] [2]
  • Removed a redundant section header in the tutorials documentation to streamline the structure.

• Newsletter Archive

  • Added the new 2.0 release announcement to the newsletter archive for visibility.

These changes ensure that users are informed about the major update, have clear installation guidance, and can easily find information about both the new and legacy versions.