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-node-rejoin.sgml

@@ -26,10 +26,10 @@
     <tip>
       <para>
         If the node is running and needs to be attached to the current primary, use
-        <xref linkend="repmgr-standby-follow">.
+        <xref linkend="repmgr-standby-follow"/>.
       </para>
       <para>
-        Note <xref linkend="repmgr-standby-follow"> can only be used for standbys which have not diverged
+        Note <xref linkend="repmgr-standby-follow"/> can only be used for standbys which have not diverged
         from the rest of the cluster.
       </para>
     </tip>
@@ -230,12 +230,13 @@
 
   <refsect1 id="repmgr-node-rejoin-pg-rewind" xreflabel="Using pg_rewind">
 
+   <title>Using <command>pg_rewind</command></title>
+
    <indexterm>
       <primary>pg_rewind</primary>
       <secondary>using with "repmgr node rejoin"</secondary>
     </indexterm>
 
-    <title>Using <command>pg_rewind</command></title>
     <para>
       <command>repmgr node rejoin</command> can optionally use <command>pg_rewind</command> to re-integrate a
       node which has diverged from the rest of the cluster, typically a failed primary.
@@ -321,7 +322,7 @@
         If <option>--force-rewind</option> is used with the <option>--dry-run</option> option,
         this checks the prerequisites for using <application>pg_rewind</application>, but is
         not an absolute guarantee that actually executing <application>pg_rewind</application>
-        will succeed. See also section <xref linkend="repmgr-node-rejoin-caveats"> below.
+        will succeed. See also section <xref linkend="repmgr-node-rejoin-caveats"/> below.
       </para>
 
     </note>
@@ -344,12 +345,13 @@
 
   <refsect1 id="repmgr-node-rejoin-caveats" xreflabel="Caveats">
 
-   <indexterm>
-      <primary>repmgr node rejoin</primary>
-      <secondary>caveats</secondary>
-    </indexterm>
-
    <title>Caveats when using <command>repmgr node rejoin</command></title>
+
+   <indexterm>
+     <primary>repmgr node rejoin</primary>
+     <secondary>caveats</secondary>
+   </indexterm>
+
    <para>
      <command>repmgr node rejoin</command> attempts to determine whether it will succeed by
      comparing the timelines and relative WAL positions of the local node (rejoin candidate) and primary
@@ -381,7 +383,7 @@
   <refsect1>
     <title>See also</title>
     <para>
-     <xref linkend="repmgr-standby-follow">
+     <xref linkend="repmgr-standby-follow"/>
     </para>
   </refsect1>
 </refentry>