bpo-33201: Modernize "Extension types" doc (GH-6337)

* bpo-33201: Modernize "Extension types" doc
* Split tutorial and other topics
* Some small fixes
* Address some review comments
* Rename noddy* to custom* and shoddy to sublist
* Fix markup

Author: pitrou

Doc/extending/index.rst

@@ -26,9 +26,11 @@ Recommended third party tools
 =============================
 
 This guide only covers the basic tools for creating extensions provided
-as part of this version of CPython. Third party tools like Cython,
-``cffi``, SWIG and Numba offer both simpler and more sophisticated
-approaches to creating C and C++ extensions for Python.
+as part of this version of CPython. Third party tools like
+`Cython <http://cython.org/>`_, `cffi <https://cffi.readthedocs.io>`_,
+`SWIG <http://www.swig.org>`_ and `Numba <https://numba.pydata.org/>`_
+offer both simpler and more sophisticated approaches to creating C and C++
+extensions for Python.
 
 .. seealso::
 
@@ -52,6 +54,7 @@ C extensions.
    :numbered:
 
    extending.rst
+   newtypes_tutorial.rst
    newtypes.rst
    building.rst
    windows.rst