replace hardcoded refs in docs with extlinks

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>

doc/tutorial/narrative-documentation.rst

@@ -91,9 +91,7 @@ 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 acts as `a target`__.
-
-__ https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#hyperlink-targets
+explicit *label* that acts as :duref:`a target <hyperlink-targets>`.
 
 For example, to reference the "Installation" subsection, add a label right
 before the heading, as follows: