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_cast.sgml

@@ -153,7 +153,7 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
     ambiguity that cannot be avoided as above.  The parser has a fallback
     heuristic based on <firstterm>type categories</firstterm> and <firstterm>preferred
     types</firstterm> that can help to provide desired behavior in such cases.  See
-    <xref linkend="sql-createtype"> for
+    <xref linkend="sql-createtype"/> for
     more information.
    </para>
   </note>
@@ -301,7 +301,7 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
   <title>Notes</title>
 
   <para>
-   Use <xref linkend="sql-dropcast"> to remove user-defined casts.
+   Use <xref linkend="sql-dropcast"/> to remove user-defined casts.
   </para>
 
   <para>
@@ -412,9 +412,9 @@ CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
   <title>See Also</title>
 
   <para>
-   <xref linkend="sql-createfunction">,
-   <xref linkend="sql-createtype">,
-   <xref linkend="sql-dropcast">
+   <xref linkend="sql-createfunction"/>,
+   <xref linkend="sql-createtype"/>,
+   <xref linkend="sql-dropcast"/>
   </para>
  </refsect1>