Extend documentation for mainnet and tetsnet

[mrLSD]

What should be done

Currently in docs directory located an example only for localnet.

Also should exist examples and how-to for NEAR blockchain networks: maintnet and testnet.

Motivation

For real-world usage, it's important to have clear instructions how-to on using and testing smart contracts (primarily Aurora Engine) for testnet. And how to use it in production for mainnet.