guide and readme - use complete examples with labels on points/edges

[redblobgames]

Rationale: the example code in the README isn't complete, so it can't be run. Provide a complete example instead, with a visualization showing what it outputs.

Use the same example in the Guide.

Change the Guide example at the top of the page to show a complete example instead of displaying a subset of the output. This means we now show half-edges without an opposite, an important half-edge case that was previously not shown in the Guide.

Add a diagram to the Guide showing the half-edge ids. Show the output of delaunay.triangles and delaunay.halfedges so that the reader can study the half-edge ids and point ids in the diagram to see how they correspond to the values in the output arrays.

Be more explicit about which code works with "new Delaunator(coords)" and which code works with "Delaunator.from(points)". Use those names, "coords" and "points" consistently.

Some of the examples here are fragile in the sense that docs/index.html makes assumptions about the examples in docs/diagrams.js. I've put <-- comments --> in those places as reminders.

Fixes #83, #76