Updated U2F chapter in the documentation
Updated U2F chapter in the documentation

Closes #2309

yacloud-io/teleport,yacloud-io/teleport,yacloud-io/teleport,gravitational/teleport,yacloud-io/teleport,yacloud-io/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,yacloud-io/teleport,gravitational/teleport,yacloud-io/teleport,gravitational/teleport