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

doc/src/sgml/filelist.sgml

@@ -83,6 +83,7 @@
 <!ENTITY arch-dev   SYSTEM "arch-dev.sgml">
 <!ENTITY bki        SYSTEM "bki.sgml">
 <!ENTITY catalogs   SYSTEM "catalogs.sgml">
+<!ENTITY system-views  SYSTEM "system-views.sgml">
 <!ENTITY geqo       SYSTEM "geqo.sgml">
 <!ENTITY btree      SYSTEM "btree.sgml">
 <!ENTITY gist       SYSTEM "gist.sgml">

doc/src/sgml/postgres.sgml

@@ -251,6 +251,7 @@ break is not needed in a wider output rendering.
 
   &arch-dev;
   &catalogs;
+  &system-views;
   &protocol;
   &sources;
   &nls;