add ModelBuilder to documentation

[williambdean]

The ModelBuilder is not in the documentation. However, there are some references and some examples that are present in the docstrings.

Steps:

• Add to the docs/source/api/index.md
• Use .. code-block:: python for examples
• Add any additional information and examples
• Better reference to the new section

[ColtAllen]

Are there instances where you anticipate users will use the ModelBuilder class directly?

[williambdean]

Are there instances where you anticipate users will use the ModelBuilder class directly?

Potentially. It is at least helpful reference for developers

To me, just referencing it other places in docs makes it docs-worthy