Fixes #4955. Move mdv sample to its own gui-cs repo

[Copilot]

Fixes

The mdv sample has been copied to https://github.com/gui-cs/mdv. This PR removes it from the library repo and adds it to the showcase.

Proposed Changes/Todos

• Delete Examples/mdv/ directory (Program.cs, README.md, mdv.csproj, launchSettings.json)
• Remove mdv project entry, build configurations, and folder nesting from Terminal.sln
• Add mdv to the showcase list in docfx/docs/showcase.md linking to https://github.com/gui-cs/mdv

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working