[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.
@pelson is correct, we use the term scalar coordinate to refer to a length one coordinate that is not associated with a dimension. If I had a length one dimension (e.g. data.shape = (3, 1, 4)), and the cube had a coord bound to dim 1, this would not be listed withe the scalar coords.