[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.
The value of (x, y, t) cubes is that they're quite similar to what readers of the document will be doing with concat in real life.

My hope was to make the examples (I tried to do the same with the merge example too) something they could relate by making the example look like what they'll be using concat with