Improve API documentation

[ponyisi]

Improve the presentation of the servicex API documentation:

• move the top-level module into its own page
• move the internals to their own file
• keep top-level servicex.app and servicex.func_adl indices
• use enum-tools to make enums look nicer

[matthewfeickert]

@ponyisi sorry that I didn't get to this today. I'll try to get a local review done on Friday before late afternoon.

[BenGalewsky]

Is there any way to make the DeliveryEnum type in the General constructor argument documentation clickable?
i.e.

Delivery – (DeliveryEnum) Specifies the delivery method for the output files.

[ponyisi]

@BenGalewsky I've tried a new variant where the "constructor" isn't documented at all for the Pydantic classes (this is the default, we only introduced that block because of trying to make command-line docstrings work nicely).