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_foreign_table.sgml

@@ -131,7 +131,7 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
       The data type of the column. This can include array
       specifiers. For more information on the data types supported by
       <productname>PostgreSQL</productname>, refer to <xref
-      linkend="datatype">.
+      linkend="datatype"/>.
      </para>
     </listitem>
    </varlistentry>
@@ -155,7 +155,7 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
       tables from which the new foreign table automatically inherits
       all columns.  Parent tables can be plain tables or foreign tables.
       See the similar form of
-      <xref linkend="sql-createtable"> for more details.
+      <xref linkend="sql-createtable"/> for more details.
      </para>
     </listitem>
    </varlistentry>
@@ -252,7 +252,7 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
      <para>
       The name of an existing foreign server to use for the foreign table.
       For details on defining a server, see <xref
-      linkend="sql-createserver">.
+      linkend="sql-createserver"/>.
      </para>
     </listitem>
    </varlistentry>
@@ -361,11 +361,11 @@ CREATE FOREIGN TABLE measurement_y2016m07
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-alterforeigntable"></member>
-   <member><xref linkend="sql-dropforeigntable"></member>
-   <member><xref linkend="sql-createtable"></member>
-   <member><xref linkend="sql-createserver"></member>
-   <member><xref linkend="sql-importforeignschema"></member>
+   <member><xref linkend="sql-alterforeigntable"/></member>
+   <member><xref linkend="sql-dropforeigntable"/></member>
+   <member><xref linkend="sql-createtable"/></member>
+   <member><xref linkend="sql-createserver"/></member>
+   <member><xref linkend="sql-importforeignschema"/></member>
   </simplelist>
  </refsect1>
 </refentry>