Fixed #923 by replacing the usage of 'ifconfig' with 'ip a' where appropriate and added a note to use 'ip a' instead of 'ifconfig' for a screencast transscript.
Use the canonical 'ip' commands to make it easier for new 'iproute2' users to understand the usage.
Merge pull request #924 from eliasp/remove-ifconfig-usage

* Documentation: replace `ifconfig` in docs with `iproute`