docs: convert diagrams from PlantUML to Mermaid

See:
- https://mermaid.js.org/syntax/sequenceDiagram.html
- https://docs.github.com/en/repositories/working-with-files/using-files/working-with-non-code-files#displaying-mermaid-files-on-github

Fix #1682

[skip ci]

Author: php-coder