Add a glossary to the documentation

More work is still needed, but this is a good start.
Co-authored-by: Corey Huinker <corey.huinker@gmail.com>
Co-authored-by: Jürgen Purtz <juergen@purtz.de>
Co-authored-by: Roger Harkavy <rogerharkavy@gmail.com>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CADkLM=eP6HOeqDjn0FdXuGRusQu4oWH_LFsKjjafmhvWD=aSpQ@mail.gmail.com

doc/src/sgml/filelist.sgml

@@ -170,6 +170,7 @@
 
 <!ENTITY limits     SYSTEM "limits.sgml">
 <!ENTITY acronyms   SYSTEM "acronyms.sgml">
+<!ENTITY glossary   SYSTEM "glossary.sgml">
 <!ENTITY color      SYSTEM "color.sgml">
 
 <!ENTITY features-supported   SYSTEM "features-supported.sgml">

doc/src/sgml/postgres.sgml

@@ -278,6 +278,7 @@
   &docguide;
   &limits;
   &acronyms;
+  &glossary;
   &color;
 
  </part>