Add content for Microsoft.ML.Tokenizers

[gewarren]

Describe the issue or suggestion

Is there active work on ML.NET and how much effort should we be putting into updating those docs?

Luis:

I'd expect minimal other than freshness updates and making sure we're not recommending to use unsupported versions of .NET in the prereqs. There's no new features we're adding at this time. We also don't expect breaking changes to existing APIs.

The only exception to that is Microsoft.ML.Tokenizers which is technically part of the ML.NET set of libraries. That's an area we don't have a lot of documentation around besides what's in the NuGet README but has received more investment in the past few months.