Convert documentation to DocBook XML

Since some preparation work had already been done, the only source
changes left were changing empty-element tags like <xref linkend="foo">
to <xref linkend="foo"/>, and changing the DOCTYPE.

The source files are still named *.sgml, but they are actually XML files
now.  Renaming could be considered later.

In the build system, the intermediate step to convert from SGML to XML
is removed.  Everything is build straight from the source files again.
The OpenSP (or the old SP) package is no longer needed.

The documentation toolchain instructions are updated and are much
simpler now.

Peter Eisentraut, Alexander Lakhin, Jürgen Purtz

doc/src/sgml/ref/cluster.sgml

@@ -57,7 +57,7 @@ CLUSTER [VERBOSE]
    <command>CLUSTER <replaceable class="parameter">table_name</replaceable></command>
    reclusters the table using the same index as before.  You can also
    use the <literal>CLUSTER</literal> or <literal>SET WITHOUT CLUSTER</literal>
-   forms of <xref linkend="sql-altertable"> to set the index to be used for
+   forms of <xref linkend="sql-altertable"/> to set the index to be used for
    future cluster operations, or to clear any previous setting.
   </para>
 
@@ -148,18 +148,18 @@ CLUSTER [VERBOSE]
     as double the table size, plus the index sizes.  This method is often
     faster than the index scan method, but if the disk space requirement is
     intolerable, you can disable this choice by temporarily setting <xref
-    linkend="guc-enable-sort"> to <literal>off</literal>.
+    linkend="guc-enable-sort"/> to <literal>off</literal>.
    </para>
 
    <para>
-    It is advisable to set <xref linkend="guc-maintenance-work-mem"> to
+    It is advisable to set <xref linkend="guc-maintenance-work-mem"/> to
     a reasonably large value (but not more than the amount of RAM you can
     dedicate to the <command>CLUSTER</command> operation) before clustering.
    </para>
 
    <para>
     Because the planner records statistics about the ordering of
-    tables, it is advisable to run <xref linkend="sql-analyze">
+    tables, it is advisable to run <xref linkend="sql-analyze"/>
     on the newly clustered table.
     Otherwise, the planner might make poor choices of query plans.
    </para>
@@ -221,7 +221,7 @@ CLUSTER <replaceable class="parameter">index_name</replaceable> ON <replaceable
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="app-clusterdb"></member>
+   <member><xref linkend="app-clusterdb"/></member>
   </simplelist>
  </refsect1>
 </refentry>