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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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5766443695d2ee82c2c4b9a23983c0af4b29711f

Modified Files
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doc/src/sgml/catalogs.sgml | 4727 ---------------------------------------
doc/src/sgml/filelist.sgml | 1 +
doc/src/sgml/postgres.sgml | 1 +
doc/src/sgml/system-views.sgml | 4750 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 4752 insertions(+), 4727 deletions(-)