docs: restructure Migration nav into Assess / Plan / Implement phases

Align with GitHub Modernization tool's three-step workflow:
- Assess: readiness, deprecation, fidelity divergences
- Plan: strategies, .NET Standard, automation, analyzers
- Implement: master pages, user controls, custom controls, etc.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz

mkdocs.yml

@@ -192,7 +192,6 @@ nav:
       - Implement:
           - Master Pages: Migration/MasterPages.md
           - User Controls: Migration/User-Controls.md
-          - Inline C# Expressions: Migration/InlineCSharp.md
           - Custom Controls: Migration/Custom-Controls.md
           - Custom Control Base Classes: Migration/CustomControl-BaseClasses.md
           - FindControl Migration: Migration/FindControl-Migration.md