Restructure README

[Fil]

This proposal (see page):

• Uses h2 and an icon everywhere, for a nice (automated) TOC on GitHub
• Duplicates the 1-line description at the top
• Defers to d3js.org for the API reference (now called Documentation)
• Adds Documentation and Examples links
• Moves Observable to the top of the Installing section

For this particular module, the reference to beeswarm is obsolete. (Though I still like the old beeswarm, and am tempted to make a notebook for it 🤓).

Once we settle on a template, this will have to be replicated to all the modules covered by d3js.org

[Fil]

hmm, maybe we don't need the icons anymore…

[Fil]

OK, I've minimized it again (preview). Also, using the D3 logo from d3/d3 — would it be any better to have a local copy?

[Fil]

I tried emojis, and while I found them useful to identify sections in the long readme, I don't like them in the simple list (in particular, the emojis for "help" and "examples" are not evocative enough).

[mbostock]

I like the emojis just to make it a little less dry. We can add them later.