fix: rdoc generation excluding MODEL_CLASS (closes #317)

[Justintime50]

Description

Rdoc was getting confused about the constant taking precedence over the class name for classes containing a MODEL_CLASS, this change tells rdoc not to document the constant which wasn't needed to begin with which removes the unnecessary MODEL_CLASS doc generation and cleans up the resulting output.

Also bumps rdoc to use newer stylized docs output.

Closes #317

Testing

See new output:

Pull Request Type

Please select the option(s) that are relevant to this PR.

• 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 not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

Also needs cleanup under the EasyPost - > Models section

[Justintime50]

Also needs cleanup under the EasyPost - > Models section

Yeah, this fixes that. Rdoc was most certainly confused about the same named files in two separate dirs. Removing the MODEL_CLASS const from docs makes everything right in the world.