Mermaid allows for simple diagrams in Markdown and now it is supported on GitHub!

Mermaid docs: https://mermaid-js.github.io/mermaid/#/README
GitHub announcement: https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/

Cover image Creative Commons:
https://www.maxpixel.net/Sea-Cartoon-Fantasy-Fish-Mirror-Mermaid-Siren-5542841