Description

• drops unmaintained m2r in favor of recommonmark, and updates sphinx version
• replaces rst-formatted narrative documentation with md-formatted equivalents
• adds auto-generated summary tables to many sections of the API Reference docs, to help users get an overview of functionality and find what they're looking for
• removes manual section references in favor of auto-generated references
• fixes and improves some docstrings throughout the code

Motivation and Context

Docs have always been a weak point; this PR takes a couple steps in the right direction.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation, and I have updated it accordingly.