Replace mermaid diagram in README.md

[erika-redding]

While the diagram renders properly on the repo's page, there's no support to render these diagrams to our gh pages site. We'll need to devise a plan for a replacement diagram or image that renders properly to both the README.md and our main gh page. Suggestions are welcome. thanks!

source: https://github.com/orgs/community/discussions/13761#discussioncomment-2873107