[WIP] feat: add mermaid rendering for diagrams

[Paul-Lez]

Written using Claude Code.

Not ready for review yet but opening the PR to get feedback on whether such an addition would be welcome.

[Vierkantor]

I think this is a good idea! Not sure about the way you implement it though: looks like our renderer uses Python-Markdown under the hood, so we can look for <pre><code class="language-mermaid">...</code></pre> instead.