PlantUML is the best tool for UML and system design. GitLab has a great integration for it: https://docs.gitlab.com/ee/administration/integration/plantuml.html. Hoping Docusauraus can support it too.
Yes, we have alternative solution by writing a custom component, but:
  1. we cannot copy'n'paste existing Markdown files
  2. it does not support multi-line code very well