Directly support docusaurus

[fredemmott]

This will basically be the same as markdown output, but pages should include a header: https://docusaurus.io/docs/en/doc-markdown

[fredemmott]

cc @JoelMarcey , @yangshun :)

[JoelMarcey]

Also cc: @endiliey

[JoelMarcey]

Just so I can context switch back on this for a sec -- this is having hhapi-doc support outputting a format consumable by Docusaurus?

[fredemmott]

Yep - basically the same as markdown mode, but with the metadata block added