[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.
You diagnose the problem by observing a massive anonymous leading dimension on your merged cube. You then don't do anything about it because there is nothing that can be done about it in Iris at the moment... @esc24 was keen that I still include this issue.