[3.12] gh-114785: Remove content from `Porting from Python2` how-to (GH-114805) (GH-115327)

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.

(cherry picked from commit 705c76d4a202f1faf41027d48d44eac0e76bb1f0)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>