Rework the .NET GitHub Copilot content and modernize/upgrade/migrate related content

[adegeo]

• Redesigned the TOC to feature what we want customers to use and rework this whole area into upgrade as a whole and not just .NET Framework.
• Split the the TOC into to pieces.
  • First is a GHCP AppMod-only product-type of TOC
  • Second is altering the upgrade guide to rework it and promote GHCP AppMod but present it with a spin as the .NET Upgrade Guide.
• Moved the index.md (which was for the .NET Framework porting guide) to framework-index.md.
• Wrote a new index.md which is a bit more generic and calls out
  • What it means to upgrade your app.
  • GitHub Copilot app modernization.
  • .NET Framework upgrades.
  • Modernize to Azure.
• Clarify the naming and scope of the "GitHub Copilot app modernization" tool, replacing older references to "upgrade for .NET" and "the tool" with more consistent terminology such as "agent".
• Updated FAQ with latest info.
• Added links to feedback.
• Removed GitHub Copilot App Modernization install article from TOC but left it for linking until that version of Visual Studio is out of support.
• Ported overview/faq info from the Azure on .NET-related content.

Fixes #48229
Fixes #47872

Important preview links

• New brief overview
• GitHub Copilot app modernization overview
• GitHub Copilot app modernization FAQ
• How to upgrade
• Landing page w/cards

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Internal previews

Toggle expand/collapse

📄 File	🔗 Preview link
docs/azure/migration/appmod/overview.md	GitHub Copilot app modernization for .NET (Preview) overview
docs/core/porting/framework-overview.md	Port from .NET Framework to .NET
docs/core/porting/github-copilot-app-modernization-faq.yml	GitHub Copilot app modernization FAQ
docs/core/porting/github-copilot-app-modernization-install.md	Install GitHub Copilot app modernization
docs/core/porting/github-copilot-app-modernization-overview.md	docs/core/porting/github-copilot-app-modernization-overview
docs/core/porting/how-to-upgrade-with-github-copilot.md	How to upgrade a .NET app with GitHub Copilot app modernization
docs/core/porting/index.md	Upgrade .NET apps overview
docs/core/porting/modernize.md	Modernize after upgrading to .NET from .NET Framework
docs/core/porting/porting-approaches.md	docs/core/porting/porting-approaches
docs/core/porting/premigration-needed-changes.md	Prerequisites to porting code
docs/core/porting/third-party-deps.md	Analyze dependencies to port code
docs/core/porting/unsupported-apis.md	Find unsupported APIs in your code
docs/core/porting/upgrade-assistant-analyze-overview.md	What is code analysis with .NET Upgrade Assistant?
docs/core/porting/upgrade-assistant-install.md	docs/core/porting/upgrade-assistant-install
docs/core/porting/upgrade-assistant-overview.md	What is .NET Upgrade Assistant?
docs/core/whats-new/dotnet-7.md	What's new in .NET 7
docs/navigate/github-copilot-modernize/index.yml	docs/navigate/github-copilot-modernize/index
docs/navigate/github-copilot-modernize/toc.yml	docs/navigate/github-copilot-modernize/toc
docs/navigate/migration-guide/index.yml	Upgrade .NET apps documentation
docs/navigate/migration-guide/toc.yml	docs/navigate/migration-guide/toc
docs/standard/analyzers/portability-analyzer.md	The .NET Portability Analyzer