Apply smooth-scroll when `prefers-reduced-motion: no-preference` (#31877)

Apply smooth-scroll when `prefers-reduced-motion: no-preference` (#31877)

* feat(reboot): apply smooth-scroll when not prefers-reduced-motion

* docs(migration): mention scroll-behavior in alpha3

* docs(accessibility): mention scroll-behavior in reduced motion section

* docs(accessibility): rewording smooth scroll sentence

* feat(variable): introduce enable-smooth-scroll variable

* docs(accessibility): reword smooth-scroll sentence

Co-authored-by: XhmikosR <xhmikosr@gmail.com>