Phase 1: Add migration decision guide and terminology glossary for WPF

[GrantMeStrength]

Summary

Adds a new migration-overview.md page and updates the WPF migration TOC as part of Phase 1 of the migration docs improvement project.

What's new

New page: dotnet-desktop-guide/wpf/migration/migration-overview.md
URL: /dotnet/desktop/wpf/migration/migration-overview
Title: Choose your WPF migration path from .NET Framework

This page provides:

• Canonical terminology definitions — establishes upgrade, migrate, modernize, port, .NET Framework, .NET, and WPF as consistent terms across all migration docs
• Decision table — helps developers choose the right path (Upgrade Assistant, CoreWCF migration, WinUI 3 rebuild) based on their app's complexity and dependencies
• Multi-project upgrade order guidance — a gap identified as causing developer confusion
• Tools table — surfaces the GitHub Copilot modernization agent alongside Upgrade Assistant

Updated: dotnet-desktop-guide/wpf/toc.yml — adds Choose your migration path as the first entry in the Migration section, so developers hit this decision guide before the tool walkthrough.

Context

This PR is part of a larger effort to improve the WPF/WinForms migration documentation based on developer feedback analysis. The existing Upgrade Assistant walkthrough (migration/index.md) is intentionally unchanged — it remains the correct Tier 1 (simple app) guide.

A companion WinForms version of this decision page is tracked for a follow-up PR.

Terminology decisions pending team review

The terminology table in this page will drive a search/replace pass across migration docs once agreed. Key decisions for the team:

• Should guide titles pin to a specific .NET version (e.g. Upgrade to .NET 8) or be evergreen (e.g. Upgrade to .NET)?
• Should port/porting be retired in docs-desktop cross-references or preserved as-is?

cc @GrantMeStrength

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Internal previews

📄 File	🔗 Preview link
dotnet-desktop-guide/wpf/migration/migration-overview.md	dotnet-desktop-guide/wpf/migration/migration-overview

[Copilot]

Pull request overview

This PR adds an entry-point WPF migration decision guide that defines standard migration terminology, and wires it into the WPF Migration navigation so it appears before the existing Upgrade Assistant walkthrough.

Changes:

• Added a new WPF migration overview page with terminology definitions, a decision table, upgrade order guidance, and a tools list.
• Updated the WPF toc.yml to surface the new page as the first Migration item.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
dotnet-desktop-guide/wpf/toc.yml	Adds the new migration overview page to the Migration section as the first entry.
dotnet-desktop-guide/wpf/migration/migration-overview.md	Introduces the new “choose your migration path” overview content and terminology glossary.

[GrantMeStrength]

Closing based on feedback from @adegeo.

tl;dr: The PR overreaches into .NET upgrade territory that Andy is actively covering in issues #2165/#2166, and the tools table leads with .NET Upgrade Assistant — which per the team's current direction should no longer be showcased as the primary recommendation.

What changes as a result:

• The WinUI 3 rebuild path (which is what Windows docs owns) is already covered in ai-modernize.md and wpf-plus-winappsdk.md. I'll add a 'should I upgrade or rebuild?' decision table there so the WinUI 3 destination is discoverable from Windows docs without duplicating the .NET upgrade mechanics.
• The in-place WPF/WinForms upgrade path (Copilot Modernization Agent, .NET terminology, upgrade mechanics) belongs in your hands alongside WinForms upgrade assistant docs need to be modernized #2165/WPF upgrade assistant docs need to be modernized #2166. I'll cross-link to your content once it's published.
• The terminology definitions in this PR's table are yours to own for docs-desktop.

Thanks for the detailed context — it saved us from shipping something that conflicts with where the team is heading. Happy to collaborate on cross-links once your sprint content is live.