Missing `getContractById` and `getContractHashes` native Management API docs

[AnnaShaleva]

neo-project/neo#2807 introduces getContractById and getContractHashes methods of native ContractManagement contract. This change was included into 3.5.0 and currently missing from the docs, we need to add the docs about it to https://github.com/neo-project/docs/tree/master/docs/en-us/reference/scapi/framework/native/ContractManagement.

Ref. neo-project/neo-devpack-dotnet#784.