[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.
We don't need 3 dimensions here. It would be good to look at the coordinates that get concatenated (let's use z rather than t to avoid needing to explain the time serialization in this section).