doc: add seealso link to src-layout vs flat-layout discussion

Add a ``.. seealso::`` cross-reference to the packaging.python.org
discussion on src layout vs flat layout, using intersphinx rather
than a raw URL.

Author: him2him2

doc/en/explanation/goodpractices.rst

@@ -150,6 +150,11 @@ which are better explained in this excellent `blog post`_ by Ionel Cristian Măr
     See :ref:`pytest vs python -m pytest` for more information about the difference between calling ``pytest`` and
     ``python -m pytest``.
 
+.. seealso::
+
+    :doc:`packaging:discussions/src-layout-vs-flat-layout`
+        The Python Packaging User Guide discusses the trade-offs between the ``src`` layout and flat layout.
+
 Tests as part of application code
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^