Update UML diagram links into documentation website

[cicirello]

Summary

Updated the UML diagram in the README, including linking classes new in version 4.0.0 to the documentation website. This is part of #302.

Types of changes

• I have read the CONTRIBUTING document.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvements to existing code, such as refactoring or optimizations (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to change)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[github-actions]

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