Documentation for using estimation mode for model development

[jpn--]

ActivitySim's estimation mode documentation needs to be expanded to better explain how to use Estimation Mode for model development, including:

• How larch.ModelGroup works to jointly estimate multiple models.
• How to manipulate the utility function of a model after it has been declared but before parameters are estimated. For advanced users, this can facilitate model development, allowing for estimating models that are not strict copies of an existing ActivitySim model spec.

[bhargavasana]

Given updated documentation in #917, can this be closed?