[docs] SceneDataset JSON

[aclegg3]

Motivation and Context

Added improved documentation for SceneDataset JSON and Attributes.

Added docstrings for python Attributes, AttributeManager, and MetadataMediator modules/classes for improved clarity.

Primary doc topics:

• SceneDataset and .scene_dataset_config.json tags
• Scene instances and .scene_instance.json tags
• User Defined metadata options ("user_defined")
  • including .ao_config.json files

How Has This Been Tested

Locally built the docs to test.

New Overview diagram and description:

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [x ] I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[Skylion007]

Nit: can the diagram be a vector graphic (such as SVG?)

[jturner65]

LGTM!