doc: convert documentation to DocBook XML

This brings the repmgr documentation build system in line with that
used by the main PostgreSQL project, and removed the restriction
that documentation must be built against PostgreSQL 9.6 or earlier.

Main formatting changes are:

 - convert empty-element tags (mainly <xref/>)
 - put <indexterm> sections in the correct location
 - correct usage of various entities.

doc/repmgr.sgml

@@ -1,7 +1,8 @@
 <!-- doc/repmgr.sgml -->
 
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.2//EN" [
-
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+          "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
+[
           <!ENTITY % version SYSTEM "version.sgml">
           %version;
 
@@ -38,7 +39,7 @@
    <para>
     &repmgr; is developed by
     <ulink url="https://2ndquadrant.com">2ndQuadrant</ulink>
-    along with contributions from other individuals and companies.
+    along with contributions from other individuals and organisations.
     Contributions from the community are appreciated and welcome - get
     in touch via <ulink url="https://github.com/2ndQuadrant/repmgr">github</ulink>
     or <ulink url="https://groups.google.com/group/repmgr">the mailing list/forum</ulink>.
@@ -128,7 +129,6 @@
  &appendix-packages;
  &appendix-support;
 
- <![%include-index;[&bookindex;]]>
- <![%include-xslt-index;[<index id="bookindex"></index>]]>
+ <index id="bookindex"></index>
 
 </book>