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

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