Simplify cross-contract calling docs

[davidsemakula]

Related PRs:

• Refactor contract ref generation and add automatic re-exporting ink#2710
• Simpler cross-contract calls example ink-examples#90

Details:

• Simplify example for using statically generated contract references by
  • Assuming user has an address for an instance of the callee contract (i.e. callee is already deployed on-chain)
  • Removing steps for manually re-exporting the contract ref of the callee (this is now automatically done by codegen since Refactor contract ref generation and add automatic re-exporting ink#2710)
  • Introduce statically generated contract refs before manually defined contract refs
• Use 20 bytes addresses (i.e. H160/ink::Address) in deployment and calling examples