Render markdown header anchors

[jackyzhen]

Currently rendering the API overview like this: https://api-doc.movio.co/org/movio.cinema/doc/api_overview

The headings don't have anchors, and so any self references to them within the document don't navigate to them.

Had a look at the mark-down library we're using. We might have to do this: remarkjs/react-markdown#69