Add hashing / verify signature sections

Helpful links
https://solidity-by-example.org/hashing/
https://solidity-by-example.org/signature/
https://github.com/FuelLabs/sway/blob/master/sway-lib-std/src/vm/evm/ecr.sw
https://github.com/FuelLabs/sway/blob/master/sway-lib-std/src/vm/evm/evm_address.sw
https://github.com/FuelLabs/sway/blob/master/sway-lib-std/src/hash.sw
https://docs.fuel.network/docs/sway/blockchain-development/hashing_and_cryptography/

Start with the linked Solidity by Example hashing and signature pages, then review the referenced Sway files: ecr.sw, evm_address.sw, and hash.sw. Add the hashing and signature sections to the examples, using the Fuel hashing and cryptography documentation as context; done means both sections are present and cover the requested topics.

Written by the indexing model from the issue text.