crucial doc updates

[jhnwu3]

This pull request primarily adds documentation for a new interpretability module, updates existing documentation for model references, and clarifies testing guidelines. The most significant changes are the addition of interpretability documentation and improvements to the organization and clarity of docs.

Documentation improvements

• Added a new section for the interpretability module in docs/api/interpret.rst, including a toctree entry for the chefer method.
• Created detailed API documentation for pyhealth.interpret.methods.chefer, describing the transformer interpretability module and its class members.
• Updated the main documentation index in docs/index.rst to include a link to the new interpretability API section.

Model documentation updates

• Renamed and updated the documentation for the SDOH model from pyhealth.models.TransformersModel to pyhealth.models.sdoh in docs/api/models/pyhealth.models.SDOH.rst.

Contribution guidelines clarification

• Added instructions specifying that all unit tests should be placed in the tests/ directory, with core functionality tests in tests/core/, to docs/how_to_contribute.rst.