Add link to docutils docs

doc/tutorial/index.rst

@@ -407,7 +407,9 @@ in the project, in this case the ``usage.rst`` you created earlier.
 
 Alternatively, you can also add a cross-reference to an arbitrary part of the
 project. For that, you need to use the :rst:role:`ref` role, and add an
-explicit *label* that can act as a target.
+explicit *label* that acts as `a target`__.
+
+__ https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#hyperlink-targets
 
 For example, to reference the "Installation" subsection, add a label right
 before the heading, as follows: