[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.
I kept them separate because they discuss slightly different subjects so my decision was two notes would be clearer and easier to read than one long note with two paragraphs.