Add documentation on Starknet Paymaster

[enitrat]

The Starknet Docs currently don't include any mention of Paymaster, which is powerful as it allows app developers to sponsor their user's transactions. The Starknet Docs should mention this as its architecture allows this easily and it's already well implemented by the ecosystem.

This is mentioned in third-party libraries already and should be mentioned in the SN docs
https://www.starknet-react.com/docs/paymaster-providers
https://starknetjs.com/docs/next/guides/account/paymaster/
https://starknetdart.dev/packages/avnu_provider