Update ML.NET docs to remove `runtime` in namespaces

[justinormont]

In dotnet/machinelearning#1956, we removed runtime from the namespaces.

Now we have to update the ML.NET example code.

For example, we moved:

• Microsoft.ML.Runtime.Data to Microsoft.ML.Data
• Microsoft.ML.Runtime.TimeSeriesProcessing to using Microsoft.ML.TimeSeriesProcessing
• Microsoft.ML.Runtime.Learners to Microsoft.ML.Learners

cc: @JRAlexander

Cross issued from ML.NET repo as both the docs and main repo need updating -- dotnet/machinelearning#1964