New API References, Migration guide and other refinements

[undavide]

Description

• Add a completely revised V4 API Reference section, automatically generated from the SDK source code
• Add a new V3 -> V4 migration guide
• Several minor refinements (including deprecation alerts, revamped landing pages, etc.)

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.