Wasm-bindgen: Add IndexedDB and web-sys example to `examples/` and guide

[fitzgen]

General info on IndexedDB: https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API

• [ ] Add an example to examples/ that shows off working with IndexedDB
• [ ] Add a subsection to "web-sys >> examples" in the guide that includes this example

[alexcrichton]

I"m gonna close this in favor of https://github.com/rustwasm/gloo/issues/68 because I think IndexedDB is more likely to be covered by gloo rather than wasm-bindgen examples

[indolering]

@alexandrestein rustwasm/gloo#68 has been closed and won't be implemented.

[Pauan]

@indolering We still plan to add an API for IndexedDB, though probably not that specific API.