Improve/docs - will merge without review since it's just quick doc changes that won't break anything.

[jhnwu3]

This pull request simplifies the why_pyhealth.rst documentation by removing detailed pipeline explanations, code examples, and some core feature descriptions, making the page more concise and focused. The installation and quick-start code snippets are also removed from the "Get Started Today" section, directing users to dedicated documentation pages instead.

Documentation simplification:

• Removed the detailed 5-stage pipeline description, step-by-step code example for mortality prediction, and the list of supported tasks, datasets, models, and modalities from the main documentation.
• Eliminated the "Core Advantages" section describing multimodal support and unified API for all healthcare data types, streamlining the page content.

Getting started section update:

• Replaced the installation command and quick-start code example with links to dedicated "Get Started" and "Install" documentation pages, making the entry point clearer and less cluttered.