v4.0.0

What's Changed

• Documentation and repo hierarchy refactoring by @sierra-moxon in #1418

Summary: 4.0.0 is a major release that includes many changes to the documentation for Biolink Model as well
as the reorganization of the repository to support the new documentation structure and comply with LinkML best
practices. The model itself has not changed significantly, but the documentation has been updated to reflect
the current state of the model, and includes new visualizations of the model, additional text-based documentation,
and a new gh-pages documentation layout.

Full Changelog: v3.6.0...v4.0.0