Struct layer API docs are missing layer.write_metadata() calls

[edmorley]

In the docs for the new struct API, it seems the cached_layer() example are missing the layer.write_metadata() calls.

For example in:
https://docs.rs/libcnb/latest/libcnb/build/struct.BuildContext.html#method.cached_layer

In the "more complex example" case (which actually needs the old metadata to compare against), the new metadata is never written, so the layer will always be invalidated.

It also feels like forgetting to include layer.write_metadata() is something end users might do too - so perhaps we need to emphasise the need to include it in as many parts of the docs as possible?