[voiceinteraction] added detailed use case to architecture draft

I split the travel use case into a generic version and a version that
references the architecture diagram. We can discuss this today. 

As we discussed last time, the plan is to finalize the architecture draft
and agree to publish it at the next call (February 13).
You may all also be interested to know that there are a couple of voice
standards presentations scheduled for SpeechTEK (April 2-29), which will be
a great opportunity to get our ideas in front of a wider audience
https://www.speechtek.com/2020/default.aspx.