chore: Add mermaid documentation

[mikelax]

Overview

• Replace entity relationship diagram image with mermaid documentation
• Clean up GitHub action tests workflow to remove unused environment variables
• BUGFIX: Update logic in the-usher.js that checks SKIP_KEYS_CHECK env variable. Values coming from process.env are always either strings or undefined. Previous logic only checked if env variable existed (could be set to any value).

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Issue Reference

Closes #

Open Questions or Items to Callout

[mikelax]

@k1ch please check out the bugfix for the SKIP_KEYS_CHECK env variable.