nixos/pam: refactor U2F, docs about u2f_keys path (#54756)

* change enableU2F option to u2f.* set
* add few u2f options (not all) to customize pam-u2f module
* document default u2f_keys locations

Co-authored-by: Tomasz Czyż <tomasz.czyz@gmail.com>
Co-authored-by: Arda Xi <arda@ardaxi.com>