[3.14] GH-100108: Add async generators best practices section (GH-141885) (#146252)

GH-100108: Add async generators best practices section (GH-141885)
(cherry picked from commit 897fa231a7b9f3b0d5a983e1d2ab37f22304c455)

Co-authored-by: Sergey Miryanov <sergey.miryanov@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>