Add example of using ``GoodCoordinates`` to obtain nested initial guess

[dpanici]

It could be useful to have a short example using the nested coordinates objective:

something like

from desc.objectives import ObjectiveFunction, GoodCoordinates


obj = ObjectiveFunction(GoodCoordinates(eq))
constraints = get_fixed_boundary_constraints(eq) + (FixThetaSFL(eq),)
eq.solve(
    objective=obj,
    constraints=constraints,
    ftol=0,
    xtol=0,
    gtol=1e-8,
    verbose=0,
    optimizer="fmintr-bfgs",
)

[dpanici]

Superseded by #1730 which is a much better way of obtaining nested initial coordiantes using map2disc