Replace ASCII diagrams with Mermaid

[lythx]

There are a few ASCII diagrams in docs (files: data-flow.md, architecture/overview.md, simulation-lifecycle.md, conversion-flow.md, adding-commands.md, geant4-wam.md, pyodide-converter.md, simulation-services.md). They should be converted to something better-looking e.g. Mermaid diagrams.