[3.13] gh-134887: Add references to `locale` module for locale-aware number formatting references in `string` module docs (GH-134888) (GH-150122)

(cherry picked from commit 47723af4e74ae1a65108837fe15795e2f70f9d02)

Co-authored-by: Stefanie Molin <24376333+stefmolin@users.noreply.github.com>