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/create_statistics.sgml

@@ -86,8 +86,8 @@ CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_na
       dependency statistics.
       If this clause is omitted, all supported statistics kinds are
       included in the statistics object.
-      For more information, see <xref linkend="planner-stats-extended">
-      and <xref linkend="multivariate-statistics-examples">.
+      For more information, see <xref linkend="planner-stats-extended"/>
+      and <xref linkend="multivariate-statistics-examples"/>.
      </para>
     </listitem>
    </varlistentry>
@@ -178,8 +178,8 @@ EXPLAIN ANALYZE SELECT * FROM t1 WHERE (a = 1) AND (b = 0);
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-alterstatistics"></member>
-   <member><xref linkend="sql-dropstatistics"></member>
+   <member><xref linkend="sql-alterstatistics"/></member>
+   <member><xref linkend="sql-dropstatistics"/></member>
   </simplelist>
  </refsect1>
 </refentry>