Add TOC to documents

[christophlocher]

Our markdown parser has an option to create a Table of Contents based on the headings of a document. We should enable this option and add toc as a key to the API response for documents.

[christophlocher]

Since we want to offer a TOC for both HTML and MD docs we have to parse it ourselves – the blackfriday parser offers a TOC function, but it only adds a html TOC on the top of the document.

My favorite solution was for us to parse the MD to HTML (or take the HTML that is already there) and then write our own solution to parse an object from the heading elements in the code.

I think the doc object of our API response should also include a toc object:

{
  title: …,
  raw: …,
  html: …,
  components: …,
  toc: [
    {
      title: "Colors",
      children: [
        { title: "Primary" },
        { title: "Secondary" }
      ]
    },
    {
      title: "Typography"
    }
  ]
}

[christophlocher]

Keeping this open, since documentation needs to be added to the website.