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/install-requirements.sgml

@@ -1,11 +1,12 @@
 <sect1 id="install-requirements" xreflabel="installation requirements">
 
+  <title>Requirements for installing repmgr</title>
+
   <indexterm>
    <primary>installation</primary>
    <secondary>requirements</secondary>
   </indexterm>
 
-  <title>Requirements for installing repmgr</title>
   <para>
     repmgr is developed and tested on Linux and OS X, but should work on any
     UNIX-like system supported by PostgreSQL itself. There is no support for
@@ -20,7 +21,7 @@
 
   <note>
    <simpara>
-    If upgrading from &repmgr; 3.x, please see the section <xref linkend="upgrading-from-repmgr-3">.
+    If upgrading from &repmgr; 3.x, please see the section <xref linkend="upgrading-from-repmgr-3"/>.
    </simpara>
   </note>
 
@@ -72,6 +73,8 @@
 
   <sect2 id="install-compatibility-matrix">
 
+    <title>&repmgr; compatibility matrix</title>
+
     <indexterm>
       <primary>repmgr</primary>
       <secondary>compatibility matrix</secondary>
@@ -81,7 +84,6 @@
       <primary>compatibility matrix</primary>
     </indexterm>
 
-    <title>&repmgr; compatibility matrix</title>
     <para>
       The following table provides an overview of which &repmgr; version supports
       which PostgreSQL version.