[DONE] Mermaid support

[mdubakov]

@gnargl

However, the lack of Mermaid support makes it unsuitable for our technical specifications. In principle, we need support for a ```mermaid code block that will render the diagram, or else a dedicated slash command, just like in other tools like Slite or Nuclino and various Markdown editors.

Mermaid support released today 🏂 December 4, 2025 / Mermaid diagrams, Collapse and expand all levels in Tables