Update documentation to be more user focused

[braingram]

Much of the current documentation describes the internals of rdm and little focus is on how a user might interact with roman files and datamodels.

The rcal documentation is a good example of user focused documentation: https://roman-pipeline.readthedocs.io/en/latest/roman/datamodels/models.html

Some items to consider:

• the focus on jwst in the current documentation isn't relevant to someone without a background working with jwst datamodels. I suggest we remove those comparisons and convert them to internal docstrings or move them to a "developer documentation" section where the information is useful to retain
• the current auto-generated datamodel pages provide no useful information: https://roman-datamodels.readthedocs.io/en/latest/api/roman_datamodels.datamodels.ImageModel.html#roman_datamodels.datamodels.ImageModel they lack
  • a link to the parent DataModel class where common methods are defined
  • a link to the underlying node (more on that below): https://roman-datamodels.readthedocs.io/en/latest/api/roman_datamodels.stnode.WfiImage.html#roman_datamodels.stnode.WfiImage
• the current auto-generated node pages are also largely unhelpful: https://roman-datamodels.readthedocs.io/en/latest/api/roman_datamodels.stnode.WfiImage.html#roman_datamodels.stnode.WfiImage they lack
  • any description of node contents
  • the uri, as a link, for the associated schema
• knowing that we are planning to add PITs schemas to rad and support them here in rdm we should consider how we want to structure documentation of those datamodels as part of these updates

[braingram]

Following up on comments on #593

• remove docs/roman_datamodels/datamodels/stnode.rst convert any useful information to docstrings for the relevant classes and modules but keep and expand on asdf.open vs rdm.open return result
• link to rad for schema information/documentation