doc/connectivity/usb: Replace references to native_posix w native_sim

Replace the references to native_posix with native_sim,
and update link names accordingly.

Background: during this release native_sim is replacing
native_posix as the main host test/development platform.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
2 files changed