Support registering mermaid icon packs

[root-expert]

Currently there's no way to register custom icon packs in mermaid.

This is described in https://mermaid.ai/open-source/config/icons.html.

The icons can live under .Site.Params.mermaid.icons object.

params:
  mermaid:
    icons:
      - name: logos
        url: https://unpkg.com/@iconify-json/logos@1/icons.json