[3.13] gh-106318: Add examples for str.casefold() and str.lower() methods (GH-142154) (#143525)

gh-106318: Add examples for str.casefold() and str.lower() methods (GH-142154)
(cherry picked from commit 51a56a3a7bd8cf05ceda65e843fb7bae40509799)

Co-authored-by: Adorilson Bezerra <adorilson@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>