Added D2 to my Markdown-Exec gallery

[pawamoy]

Markdown-Exec is an extension for Python-Markdown that allows to execute code blocks in Markdown pages 🙂

I added an example that shows how to generate a D2 diagram in SVG to my gallery of examples: https://pawamoy.github.io/markdown-exec/gallery/#with-d2.

That's it!
Love D2, really nice project!