Add warning when using packages.config for MVVM Toolkit

[Sergio0694]

Closes #695

This PR adds a warning from the imported .targets file if using the MVVM Toolkit via packages.config.
Also adds warning codes and help links to all warnings emitted by the bundled .targets file.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions