added tutorial, racket categories
relabeled some other categories
normalized manual names: de-bolded, changed some names
original commit: 1b702a2ae32aba835f7467ced6b203902db0ccd9
Also, updates some of the mzlib files to point at `racket/*' libraries
rather than to `scheme/*' ones.
original commit: ac26fe75546b5182d78f18c2cd882f0f440849e2