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/explain.sgml

@@ -260,7 +260,7 @@ ROLLBACK;
     The command's result is a textual description of the plan selected
     for the <replaceable class="parameter">statement</replaceable>,
     optionally annotated with execution statistics.
-    <xref linkend="using-explain"> describes the information provided.
+    <xref linkend="using-explain"/> describes the information provided.
    </para>
  </refsect1>
 
@@ -276,7 +276,7 @@ ROLLBACK;
    the <link linkend="autovacuum">autovacuum daemon</link> will take care
    of that automatically.  But if a table has recently had substantial
    changes in its contents, you might need to do a manual
-   <xref linkend="sql-analyze"> rather than wait for autovacuum to catch up
+   <xref linkend="sql-analyze"/> rather than wait for autovacuum to catch up
    with the changes.
   </para>
 
@@ -450,7 +450,7 @@ EXPLAIN ANALYZE EXECUTE query(100, 200);
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-analyze"></member>
+   <member><xref linkend="sql-analyze"/></member>
   </simplelist>
  </refsect1>
 </refentry>