DOCS: Setup.py is now an optional file. (#659)

Setup.py is now an optional file.

Author: MaxJPRey

doc/source/packaging/structure.rst

@@ -307,6 +307,9 @@ for build-system backend tools commonly used in the Python ecosystem:
 
 .. include:: code/pyproject_code.rst
 
+Optional files
+==============
+
 The ``setup.py`` file
 ---------------------