Added ACF interface for defining custom subnav menu items

[cjg89]

Description

See title

Motivation and Context

Resolves #31

How Has This Been Tested?

Reviewed in Dev.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires an update to the documentation.
• [ ] I have updated the documentation accordingly.