pgsql: doc: move system views section to its own chapter

doc: move system views section to its own chapter

Previously it was inside the system catalogs chapter.

Reported-by: Peter Smith

Discussion: /messages/by-id/CAHut+PsMc18QP60D+L0hJBOXrLQT5m88yVaCDyxLq34gfPHsow@mail.gmail.com

Backpatch-through: 15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/64d364bb39cbb1d97ab6dbd6b873a1016a261d4b

Modified Files
--------------
doc/src/sgml/catalogs.sgml | 4741 ---------------------------------------
doc/src/sgml/filelist.sgml | 1 +
doc/src/sgml/postgres.sgml | 1 +
doc/src/sgml/system-views.sgml | 4750 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 4752 insertions(+), 4741 deletions(-)