Adds early-beta framing to every user-facing surface where native Windows
is introduced — landing page install block, Installation page, Windows
(Native) guide, contributor notes, and README. Sets expectations that the
path installs and runs but hasn't been road-tested as broadly as POSIX,
and points users who want maximum stability at WSL2 instead.
Follow-up to #21561 (native Windows support) and #22089 (Windows docs).