doc: document auto-generated <ContractType>Ext structs and their usage

[dj8yfo]

not very clear where to put <ContractType>Ext doc itself

EDIT: it should be a sub-heading under #[near] macro documentation, peer (at the same heading level as) to attributes such as #[private] and what not

This task should describe, that this structure is created for all contracts, and what's the purpose of each method,
and how its methods signatures can be looked up on generated cargo doc --lib for contract

#[must_use]
pub struct CrossContractExt {
    pub(crate) account_id: ::near_sdk::AccountId,
    pub(crate) deposit: ::near_sdk::NearToken,
    pub(crate) static_gas: ::near_sdk::Gas,
    pub(crate) gas_weight: ::near_sdk::GasWeight,
}
impl CrossContractExt {
    pub fn with_attached_deposit(mut self, amount: ::near_sdk::NearToken) -> Self {
        /// ...
    }
    pub fn with_static_gas(mut self, static_gas: ::near_sdk::Gas) -> Self {
        /// ...
    }
    pub fn with_unused_gas_weight(mut self, gas_weight: u64) -> Self {
        /// ...
    }
}
impl CrossContract {
    /// API for calling this contract's functions in a subsequent execution.
    pub fn ext(account_id: ::near_sdk::AccountId) -> CrossContractExt {
        /// ....
    }
}
impl CrossContractExt {
    pub fn contract_source_metadata(self) -> ::near_sdk::Promise {
        /// ...
    }
}
impl CrossContractExt {
    pub fn method_one(self, n: u32) -> ::near_sdk::Promise {}

    pub fn method_two(self, n: u32) -> ::near_sdk::Promise {
        /// ...
    }
}

links:

1. <ContractType>Ext => [near_sdk::Promise]
2. anchor [near#inner-workings-of-nearcontract_state-macro-on-structenum] => <ContractType>Ext (added in scope of doc: #[near(contract_state)] in-depth pass #1307)
3. anchor [near#inner-workings-of-near-macro-on-impl-block] => <ContractType>Ext (added in scope of doc: #[near(contract_state)] in-depth pass #1307)

[TobieTom]

Hola. I'm Tobias . I've already contributed to some projects concerning their documentation and would like to add this to my pool of experience plus I recently learned rust and cairo