Component for table of contents

[nokome]

It would be useful, particularly for long documents, to have a table of contents generated based on the heading structure of the HTML document. The main requirements for this:

• able to build a TOC from h1, h2 etc tags alone
• easily able to be styled to fit with the theme it is applied within
• indicates the current section the document the viewport is in
• light weight

A good, pre-existing solution appears to be https://tscanlin.github.io/tocbot/. We should probably use that, at least to start with.

[nokome]

After discussion with @jwijay and @alex-ketch I've moved this from thema to here. This component will still be used in thema (in the stencila theme, and possibly others).

I'd like to put this and #11 as highest priority for design and development because they are needed for the documentation that we are generating for encoda and other repos (which use the stencila theme).

[nokome]

Some inspo for this from HackMD. I like it's combination of minimalism, interactivity (tells you where you are), responsiveness (only appears on wide screens) and utility (Back to top etc).

[nokome]

Closing as we now have this https://github.com/stencila/designa/tree/master/packages/components/src/components/tableOfContents