Migrate Writers: XmlDocWriter that is responsible for writing the example doc file

[jorgerangel-msft]

The XmlDocWriter class should be migrated and stripped of any branding.

[m-nash]

We should decide if this is a special writer case or if expressions should be augmented to handle this. Leaning towards leaving this as a special writer case.

[JoshLove-msft]

This is specific to the samples XML file. Should the current XmlDocExpressions be used for this file, or do we need a separate type to do this. This is scoped to Azure-only, as we won't emit samples in the unbranded scenarios for now. This could be a flag in the future.

[JoshLove-msft]

@ArcturusZhang fyi