UX Guidelines

[undavide]

Description

Add a new version of the UX Guidelines to the Guides > Design > UX Guidelines following Matthew Ladner's new Information Architecture.

Of the two proposals (bare conversion of the existing Guidelines from Adobe XD -> Markdown, and a revised TOC) this PR implements the latter.

Please note:

• Design > Overview and Design > Best practices have been kept the same.
• Design > User Interface Guide (stripped of the XD content) has been renamed to "Implementation guide", as it's the only section that contains code snips. New versions of the UX guidelines may embed snippets in each section.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

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 change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.