Add documentation and a test for depending on zerocopy and zerocopy-derive separately

We allow users to depend on zerocopy and, instead of enabling the `derive` feature, depend on zerocopy-derive directly. As demonstrated by [this comment](https://github.com/google/zerocopy/pull/505#discussion_r1370189353), doing this can have unexpected and bad ergonomics.

We should:
- add a test that makes sure the ergonomics of this use case are acceptable
- if we can't make the ergonomics as good as those of the `derive` feature, document what users must do in this case