Adding 4 smallerpdf files beside one giant 1600+ pages Salt-all.pdf

The All-in-on Salt-all.pdf is about 1600 pages.  Goal is to let people have option to print out salt doc in 3 smaller books, the 4th book(Reference section) along is around 1000 pages. Please review this pull, it was a quick cut and chop. 
Also move xelatex checking inside "make pdf" target,so "make html" won't break.