Add a show_docs function that lets the user see the docs for a specific property of a wrapped struct

[JamieMair]

Some structs like Model or Data have a lot of properties and it can be inconvenient to find the right documentation. The show_docs function will let the user see the documentation in the REPL:

julia> show_docs(Model, :nq)
Model.nq: number of generalized coordinates = dim(qpos)

or if you have an object:

julia> model, data = MuJoCo.sample_model_and_data();
julia> show_docs(model, :nq)
Model.nq: number of generalized coordinates = dim(qpos)

It would be nice to use a macro like @showdoc model.nq, but both approaches are essentially the same and the function is easier to implement.

This feature should be implemented for:

• mjData
• mjModel
• mjStatistics
• mjOption
• Possibly for named access structs, these could always be forwarded to the appropriate parent struct.