handbook basics: add information about GPT partitions

Generalize section on disk partitions by adding information on using
GPT partitions as well as MBR, rearranging some of the material to
try to make it flow better. Use "BSD partition" to describe the
bsdlabel-based partitions inside a slice. Add section headings for
file systems and partitions. Other touch-ups while here.

Reviewed by: carlavilla
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D38137