Reorganize and rewrite examples

This commit is a large-ish scale reorganization of our examples. The
main goal here is to have a dedicated section of the guide for example,
and all examples will be listed there. Each example's `README` is now
just boilerplate pointing at the guide along with a blurb about how to
run it.

Some examples like `math` and `smorgasboard` have been deleted as they
didn't really serve much purpose, and others like `closures` have been
rewritten with `web-sys` instead of hand-bound bindings.

Overall it's hoped that this puts us in a good and consistent state for
our examples, with all of them being described in the guide, excerpts
are in the guide, and they're all relatively idiomatically using
`web-sys`.

examples/webgl/index.js

@@ -1,5 +1,5 @@
 // For more comments about what's going on here, check out the `hello_world`
 // example.
-import('./wasm_bindgen_webgl_demo').then(webgl => {
+import('./webgl').then(webgl => {
   webgl.draw();
 });