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-standby-clone.sgml

@@ -85,12 +85,13 @@
   </refsect1>
 
   <refsect1 id="repmgr-standby-clone-recovery-conf">
-   <indexterm>
+    <title>Customising recovery.conf</title>
+    <indexterm>
      <primary>recovery.conf</primary>
      <secondary>customising with &quot;repmgr standby clone&quot;</secondary>
-   </indexterm>
+    </indexterm>
+
 
-   <title>Customising recovery.conf</title>
    <para>
      By default, &repmgr; will create a minimal <filename>recovery.conf</filename>
      containing following parameters:
@@ -142,7 +143,7 @@
        We recommend using <ulink url="https://www.pgbarman.org/">Barman</ulink> to manage
        WAL file archiving. For more details on combining &repmgr; and <application>Barman</application>,
        in particular using <varname>restore_command</varname> to configure Barman as a backup source of
-       WAL files, see <xref linkend="cloning-from-barman">.
+       WAL files, see <xref linkend="cloning-from-barman"/>.
      </para>
    </note>
 
@@ -186,12 +187,13 @@
 
   <refsect1 id="repmgr-standby-create-recovery-conf">
 
+   <title>Using a standby cloned by another method</title>
+
    <indexterm>
      <primary>recovery.conf</primary>
      <secondary>generating for a standby cloned by another method</secondary>
    </indexterm>
 
-   <title>Using a standby cloned by another method</title>
    <para>
      &repmgr; supports standbys cloned by another method (e.g. using <application>barman</application>'s
      <command><ulink url="http://docs.pgbarman.org/release/2.5/#recover">barman recover</ulink></command> command).
@@ -296,7 +298,7 @@
         <term><option> --recovery-conf-only</option></term>
         <listitem>
           <para>
-            Create <filename>recovery.conf</filename> file for a previously cloned instance. &repmgr 4.0.4 and later.
+            Create <filename>recovery.conf</filename> file for a previously cloned instance. &repmgr; 4.0.4 and later.
           </para>
         </listitem>
       </varlistentry>
@@ -361,7 +363,7 @@
   <refsect1>
     <title>See also</title>
     <para>
-      See <xref linkend="cloning-standbys"> for details about various aspects of cloning.
+      See <xref linkend="cloning-standbys"/> for details about various aspects of cloning.
     </para>
   </refsect1>
 </refentry>