Blazor Desktop cleanup tasks for preview 3

[Eilon]

• Copyright headers - Eilon
• Review general public API surface - Eilon
• XML docs - Eilon
• Exclude WPF/WinForms build from non-Windows platforms. See if what we have just works already. If not, talk to Will/Doug - Javier
• Public API signatures test/TXT file - Javier
• WebView2 NuGet failure - Eilon
• Review TODO comments - Steve
• Runs on CI - everyone keep an eye, but post a message here if you're looking at something so we don't overlap
• Make sure the right packages are actually marked as shipping and show up in nightly feeds (at minimum, this means Microsoft.AspNetCore.Components.WebView - the WinForms/WPF ones are TBD) - Javier once plan confirmed
• Manual test/verification pass. Can we make BasicTestApp run inside the webview and manually click through all the scenarios? What about a bigger app like CarChecker? - Steve
• Some kind of getting started content - Eilon (will do this post-Preview 3 when we have .NET MAUI scenarios working)
  • Written guide (maybe blog post, maybe docs)
  • Official sample projects (maybe on asplabs)

[Eilon]

Public API surface was already reviewed and quite minimal to begin with, so that's done for now.

[Eilon]

@javiercn - once you're done marking the packages as shipping (WPF/WinForms and also WebView2), go ahead and close this as done!