[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 wouldn't describe it as incomplete coords (incomplete data, perhaps), but nevertheless, this is a huge problem for our users. Just yesterday I trying to help a user with missing realisations, causing the time and realisation coordinate to share a single, long, anonymous dimension. I find this confusing and I've seen it countless times.