Add site semantics to weld constraint. Fixes #1896

PiperOrigin-RevId: 675112981
Change-Id: I435da874097f1b0399af25f0da6841826706fd4d

Author: thowell

dm_control/mjcf/schema.xml

@@ -1862,6 +1862,8 @@
             <attribute name="body2" type="reference" reference_namespace="body"/>
             <attribute name="relpose" type="array" array_type="float" array_size="7"/>
             <attribute name="anchor" type="array" array_type="float" array_size="3"/>
+            <attribute name="site1" type="reference" reference_namespace="site"/>
+            <attribute name="site2" type="reference" reference_namespace="site"/>
             <attribute name="torquescale" type="float"/>
           </attributes>
         </element>