Create the initial draft documentation for DataMapper

[fbolton]

The DataMapper component is already quite feature rich. The scope of this doc issue is to catch up with the functionality implemented so far, by creating an initial draft of the DataMapper doc. At that point, this issue can be closed and new issues will be created to track subsequent improvements to the DataMapper.

[nichmoore]

I started to look at this and tried to create a User experience document. However, in the last couple of days the Data Mapper function seems to have disappeared. The option to create a data mapper step seems to have gone from the list of options.
I'll keep watching to see if it comes back.