SIP014 support: annotation extraction for documentation

[lgalabru]

stacksgov/sips#32 still needs to be formulated and specified, but working on the tooling implementation beforehand could make sense and could help us come up with a good specification.

[lgalabru]

Note: we would probably need a dedicated pass for these comments - for instance we would be emitting warnings if the doc is missing arguments, or arguments are misspelled, or type return not matching

[obycode]

This probably depends on #464.

[smcclellan]

Closing due to lack of activity. Please create a new ticket (or reopen) if still relevant.