Initial publish of custom controls and designers

[adegeo]

Fixes #1632

This PR is for an initial publish of content. The content here can be linked to from blogs/old articles/redirects. The next push on this content will be to finish it.

@KlausLoeffelmann I probably made a few mistakes in the .NET Framework diff article and the VS Designer Overview article. Please add any suggestions you want to rewrite areas, reexplain things, etc. I'm working from limited experience in this area and it took me a bit of time, blog reading, book reading, and thinking, to get up-to-speed on this.

• Custom controls
  • Overview
  • Extend an existing control
  • User controls
    • Overview
  • Visual Studio designer support
    • Overview
    • Designer changes from .NET Framework

Todo

• Redirects
• Review @KlausLoeffelmann

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Internal previews

📄 File	🔗 Preview link
dotnet-desktop-guide/net/winforms/controls-design/designer-differences-framework.md	dotnet-desktop-guide/net/winforms/controls-design/designer-differences-framework
dotnet-desktop-guide/net/winforms/controls-design/designer-overview.md	dotnet-desktop-guide/net/winforms/controls-design/designer-overview
dotnet-desktop-guide/net/winforms/controls-design/extend-existing.md	dotnet-desktop-guide/net/winforms/controls-design/extend-existing
dotnet-desktop-guide/net/winforms/controls-design/overview.md	dotnet-desktop-guide/net/winforms/controls-design/overview
dotnet-desktop-guide/net/winforms/controls-design/usercontrol-overview.md	dotnet-desktop-guide/net/winforms/controls-design/usercontrol-overview
dotnet-desktop-guide/net/winforms/controls/custom-painting-drawing.md	Painting and drawing on controls (Windows Forms .NET)

[KlausLoeffelmann]

Hey, thanks for that, and sorry for the delay.
Had a couple of thoughts and questions, but it's REALLY impressive what you did in such a short time. Glad also talk to about with you in person, if you like!
Really appreciate your work!!