Support Mermaid for Diagramming

[seshrs]

GitHub now supports embedding Mermaid diagrams. (Source)

Primer Spec should support them too! (The integration doesn't look too complex 😄 )

Here's an example Mermaid diagram:

  graph TD;
      A-->B;
      A-->C;
      B-->D;
      C-->D;

[seshrs]

Ha, looks like GitHub doesn't change the diagram colors in dark mode. I think Primer Spec can do better than that! 😉

Edit: They do change colors in dark mode, but it seems buggy/laggy.