[contracts] Add docs generator for the contracts API to the `#[define_env]` macro (#13032)

* macro to expand traits for host functions documentation

* other way: same Doc trait in seal modules

* added docs for macro, and remove `doc` attribute

* fmt

* Apply suggestions from code review

Co-authored-by: Alexander Theißen <[email protected]>

* make docs to be generated into re-exported `api_doc` module; fix
unrelated elder docs;

* make it compile without `doc` attr passed to macro

* make alias functions indicated explicitly in docs

* tidy up docs

* refactored a bit

* macro to auto-add doc warning for unstable functions

* invoke macro with no doc generation by default

* addressed review comments

* hide api_doc module behind cfg(doc)

Co-authored-by: Alexander Theißen <[email protected]>