Move Tile Cache how-to into System Resources Preferences section

Move the "How to Set Your Tile Cache" essay from the "Enrich my GIMP" [sic!] section which collects rather random topics under an unhelpful umbrella into a subsection of "Preferences Dialog > System Resources" as that's where this single preference option is covered.

Also rename the Docbook file as having three "setup.xml" files isn't particularly descriptive.

Generally speaking, the GIMP user help has a rather academical concept of keeping general concepts and how-tos and tutorials separate from the "Function Reference" area. In reality this leads to triplicated documentation scattered across random places.