Make sure all code examples containing an entry point use library feature

[aumetra]

Follow-up to #57 (comment)

We should probably enforce the convention of having a library feature in contracts by putting them explicitly into every code sample and reinforcing that this is how it should be done.

Also, as @chipshort mentioned, people will probably copy-paste from the docs a lot, so having that library feature everywhere means that they will follow best practices.

---

One open question is how to enforce this in CI. An idea I had was a simple CI step that greps across the extracted tests for ^#\[entry_point\] and errors out when it gets hit (something like that)