[.NET Migration Guide Revamp] [Phase 1] Move content 'Determine Portability' content from 'Port libraries' page.

[StephenBonikowsky]

For details on revamp see: #23561
New TOC: #23215

• Determine portability (MOVE) This section needs to be moved to a new location in the new TOC and updated as needed.

• name: Migration guide
  items:

  • name: Overview
    href: ../core/porting/index.md
  • name: General Information
    items:
  • name: Pre-Migration
    items:
    • name: Analyze & Asses the portability of your project
      items:
      • name: Unsupported APIs
        href: ../core/porting/unsupported-apis.md

[StephenBonikowsky]

Completed with #24630