Add events that help do startup performance investigations

This issue one part of a larger request being tracked in https://github.com/dotnet/runtime/issues/64993

When developers want to improve the startup time of their applications, a common approach would be to run the app while taking an ETW trace, identify the period of time that corresponds to app startup, and then analyze the app behavior during that window. It would be helpful to have one or more canonical events that indicate when startup is complete however WPF app developers would usually define that. I'm not a WPF expert but I could imagine startup might be considered complete when the first window has been displayed and is responsive to user input.

The request here is:
1) If useful startup events already exist, help make that information discoverable.
I found a few references to wpf etw events in stackoverflow and old blog posts when I searched, but the links that looked like they would be the most informative were all dead. For comparison the .NET runtime documents relevant ETW events in the [official docs](https://docs.microsoft.com/en-us/dotnet/fundamentals/diagnostics/runtime-events)
2) If no suitable events exist, add and document them.