Document how to enable compilation caching in the embedding API

Wasmtime has a compilation cache to avoid recompiling the same module multiple times. In the embedding API, it's not enabled by default; users must call [`cache_config_load_default`](https://docs.rs/wasmtime/0.21.0/wasmtime/struct.Config.html#method.cache_config_load_default). I wasn't able to find this mentioned in the guide or in any of the in-tree examples.