[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.
Also, thinking about it, having three equal-length (x, y, t) cubes means it directly connects to the diagram that follows. If I change this I'd have to change the diagram too for the sake of consistency, which would seem excessive effort for an example that does at least do its job.