[DOC]: Merge and concatenate chapter for user guide

This adds a chapter on merge and concatenate to the Iris user guide. The chapter describes what merge and concatenate do, the differences between the two and hence the situation in which each should be used. The chapter also contains a section on issues commonly encountered with merge and concatenate and, where possible, provides the solution to avoid the problem occurring.
Perhaps a warning is a little drastic! I came across this point in a support ticket when I was researching common problems with merge and concat. I guess the point is that deleting the _entire_ attributes dictionary is a bit sledgehammer when it might only be a single item that doesn't line up. That and the fact that it removes valuable metadata from the cube.