doc: Update installation instructions for Zephyr SDK 0.16.0

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>