This commit updates the Zephyr SDK installation instructions for the
Zephyr SDK 0.16.0 release.
Note that the distribution bundle archive format has been changed from
`tar.gz` to `tar.xz` for Linux and macOS, and from `zip` to `7z` for
Windows.
Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>