remove crosswalk section

[brandonopened]

http://docs.opensalt.org/en/2.2/#id77

Section 6.6.3 is pretty much a rehash of making associations all over again? Unsure why it is there. Permission to delete it with perhaps a reference in the "Make associations" section that making associations is also called making a crosswalk when between two documents.

[jmarks]

Hmmm, but this is the other way around to make a crosswalk between two other frameworks. We could certainly reference the generally make association part as being the same.

BTW, do we have any automated tests for this?