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`.

guide/src/contributing/web-sys/overview.md

@@ -31,7 +31,7 @@ The only thing `src/lib.rs` does is include the bindings generated at compile
 time in `build.rs`. Here is the whole `src/lib.rs` file:
 
 ```rust
-{{#include ../../../crates/web-sys/src/lib.rs}}
+{{#include ../../../../crates/web-sys/src/lib.rs}}
 ```
 
 ### Cargo features