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/upgrading-repmgr.sgml

@@ -1,10 +1,10 @@
 <chapter id="upgrading-repmgr" xreflabel="Upgrading repmgr">
+ <title>Upgrading repmgr</title>
 
  <indexterm>
   <primary>upgrading</primary>
  </indexterm>
 
- <title>Upgrading repmgr</title>
 
  <para>
   &repmgr; is updated regularly with minor releases (e.g. 4.0.1 to 4.0.2)
@@ -13,11 +13,12 @@
  </para>
 
  <sect1 id="upgrading-repmgr-extension" xreflabel="Upgrading repmgr 4.x and later">
+  <title>Upgrading repmgr 4.x and later</title>
+
   <indexterm>
    <primary>upgrading</primary>
    <secondary>repmgr 4.x and later</secondary>
   </indexterm>
-  <title>Upgrading repmgr 4.x and later</title>
   <para>
     From version 4, &repmgr; consists of three elements:
      <itemizedlist spacing="compact" mark="bullet">
@@ -63,11 +64,12 @@
   </important>
 
   <sect2 id="upgrading-minor-version" xreflabel="Upgrading a minor version release">
+	<title>Upgrading a minor version release</title>
+
 	<indexterm>
 	  <primary>upgrading</primary>
 	  <secondary>minor release</secondary>
 	</indexterm>
-	<title>Upgrading a minor version release</title>
 
     <para>
       The process for installing minor version upgrades is quite straightforward:
@@ -118,11 +120,13 @@
   </sect2>
 
   <sect2 id="upgrading-major-version" xreflabel="Upgrading a major version release">
+	<title>Upgrading a major version release</title>
+
 	<indexterm>
 	  <primary>upgrading</primary>
 	  <secondary>major release</secondary>
 	</indexterm>
-	<title>Upgrading a major version release</title>
+
 	<para>
 	  &quot;major version&quot; upgrades need to be planned more carefully, as they may include
 	  changes to the &repmgr; metadata (which need to be propagated from the primary to all
@@ -205,11 +209,12 @@ ALTER EXTENSION repmgr UPDATE</programlisting>
   </sect2>
 
   <sect2 id="upgrading-check-repmgrd" xreflabel="Checking repmgrd status after an upgrade">
+	<title>Checking repmgrd status after an upgrade</title>
+
 	<indexterm>
 	  <primary>upgrading</primary>
 	  <secondary>checking repmgrd status</secondary>
 	</indexterm>
-	<title>Checking repmgrd status after an upgrade</title>
 	<para>
       From <link linkend="release-4.2">repmgr 4.2</link>, once the upgrade is complete, execute the <command><link linkend="repmgr-daemon-status">repmgr daemon status</link></command>
       command (on any node) to show an overview of the status of &repmgrd; on all nodes.
@@ -218,6 +223,8 @@ ALTER EXTENSION repmgr UPDATE</programlisting>
  </sect1>
 
  <sect1 id="upgrading-and-pg-upgrade" xreflabel="pg_upgrade and repmgr">
+  <title>pg_upgrade and repmgr</title>
+
   <indexterm>
    <primary>upgrading</primary>
    <secondary>pg_upgrade</secondary>
@@ -225,7 +232,6 @@ ALTER EXTENSION repmgr UPDATE</programlisting>
   <indexterm>
     <primary>pg_upgrade</primary>
   </indexterm>
-  <title>pg_upgrade and repmgr</title>
 
   <para>
     <application>pg_upgrade</application> requires that if any functions are
@@ -265,12 +271,13 @@ ALTER EXTENSION repmgr UPDATE</programlisting>
 
 
  <sect1 id="upgrading-from-repmgr-3" xreflabel="Upgrading from repmgr 3.x">
+  <title>Upgrading from repmgr 3.x</title>
+
   <indexterm>
    <primary>upgrading</primary>
    <secondary>from repmgr 3.x</secondary>
   </indexterm>
 
-  <title>Upgrading from repmgr 3.x</title>
   <para>
    The upgrade process consists of two steps:
    <orderedlist>
@@ -383,7 +390,7 @@ ALTER EXTENSION repmgr UPDATE</programlisting>
         to the server configured in Barman (in &repmgr; 3, the deprecated
         <literal>cluster</literal> parameter was used for this);
         the physical Barman hostname is configured with
-        <literal>barman_host</literal> (see <xref linkend="cloning-from-barman-prerequisites">
+        <literal>barman_host</literal> (see <xref linkend="cloning-from-barman-prerequisites"/>
           for details).
       </para>
     </note>