Add custom converter spec detailed as example in the manual

[dukeraphaelng]

Description

• Add spec demonstrating working custom converter detailed in the manual

Motivation and Context

• To clarify custom converter working in action

How Has This Been Tested?

• Spec added in spec/model/converters/custom_converter_spec

• All specs passed

Types of changes

• 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)

• Manual of usage of the new feature.

Checklist:

• My code follows the code style of this project. bin/ameba ran without alert.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.