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/import_js/src/lib.rs

@@ -19,7 +19,7 @@ extern "C" {
     fn render(this: &MyClass) -> String;
 }
 
-// Import `console.log` so we can log something to the console
+// lifted from the `console_log` example
 #[wasm_bindgen]
 extern "C" {
     #[wasm_bindgen(js_namespace = console)]