doc: move system views section to its own chapter

Previously it was inside the system catalogs chapter.

Reported-by: Peter Smith

Discussion: https://postgr.es/m/CAHut+PsMc18QP60D+L0hJBOXrLQT5m88yVaCDyxLq34gfPHsow@mail.gmail.com

Backpatch-through: 15

Author: bmomjian