Convert diagrams to Mermaid

[php-coder]

Where possible, let's convert our diagrams from PlantUML to Mermaid. The benefit is that the latter is natively supported by GitHub (see https://docs.github.com/en/repositories/working-with-files/using-files/working-with-non-code-files#displaying-mermaid-files-on-github). This simplifies their maintenance and also doesn't require to use a separate branch (see #1495).