gh-114785: Remove content from Porting from Python2 how-to (#114805)

Keep the page though, because people might still rely on it (the traffic shows that they do).
Instead of our own manual we now give links to the 3rd-party ones.

Co-authored-by: Hugo van Kemenade <[email protected]>

Author: sobolevn

Doc/howto/index.rst

@@ -13,7 +13,6 @@ Currently, the HOWTOs are:
 .. toctree::
    :maxdepth: 1
 
-   pyporting.rst
    cporting.rst
    curses.rst
    descriptor.rst