Use Mermaid instead of drawio for diagrams

[sklivvz]

Use mermaid instead of draw.io for diagrams (mermaid allows saving diagrams as text inside markdown)

[aaubry]

@sklivvz I don't think we should use mermaid for the diagrams because:

• Many diagrams cannot be properly done using textual languages. I've tried many times but it just gets in your way.
• Github doesn't render mermaid diagrams so we would need to export them as images anyway.