Autogenerated docstrings for @version-generated record types

[omus]

It would be nice to provide some autogenerated docstrings for record types. Usually when the a Legolas record type is given a docstring the code used to generate the schema version is just copied into the docstring making maintaining the docstring annoying. Most likely we'll want to autogenerate a default docstring for Legolas record types but provide functionality such that it's easy to generate the docstring. One proposal for how we can handle this can be seen in: beacon-biosignals/Onda.jl#137 (comment)

Split out from: #10

[omus]

Quite possibly we can use DocStringExtensions.jl to provide most of the functionality we want. If supported we could write the following:

"""
    AnnotationV1

My amazing description.

# Fields

$(TYPEDFIELDS)
"""
@version AnnotationV1 begin
    "The UUID identifying the recording with which the annotation is associated."
    recording::UUID = UUID(recording)

    "The UUID identifying the annotation."
    id::UUID = UUID(id)

    "The annotation's time span within the recording."
    span::TimeSpan = TimeSpan(span)
end

This would require a change to the @version macro to handle the addition of docstrings on fields. Additionally, this would provide us a way of automatically documenting the RHS of the fields. We would need to provide our own docstring extension to provide this functionality.