add UML page

[yardasol]

Summary of changes

This PR adds a page describing UML diagrams and points to some tools for generating them.

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)
• I have read the CONTRIBUTING document.
• 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.

Associated Issues and PRs

• Issue: #

Associated Developers

• Dev: @

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