Update TimeProvider documentation to mention how it should be added to the DI

[bfondu]

There appears to be very little documentation on the new TimeProvider functionality. I had to go google around to see how this thing was expected to be registered in the DI.

Eventually I did find that I had to do this:
services.TryAddSingleton(TimeProvider.System);

Sadly, this doesn't seem to be documented anywhere. It might be good to at least mention it as a remark on the System.TimeProvider page.

Additionally, I think it would make sense to provide an extension method on IServiceCollection along the lines of AddTimeProvider(), like for example AddHttpContextAccessor() and AddMemoryCache().

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