Update guide: Making cross-contract calls

[ElliotFriend]

https://developers.stellar.org/docs/build/guides/conventions/cross-contract

[ElliotFriend]

The cross-contract guide got a substantive review in #2229 (Feb 2026) and is current — contractimport, try_ error handling, and CLI commands all check out. Closing as completed. (One small thing spotted during verification: the stellar contract bindings rust command on that page is missing its --network value — captured in #2580.)