Add uninstall option and write uninstall howto

The guide to uninstall is missing, users are requesting it

EDIT: Added on 2022-05-28
see the end of page:
https://mpdroot.jinr.ru/running-mpdroot-on-local-machine-using-cvmfs/

./nica-init.sh uninstall

Uninstalls cvmfs, removes toolbox container & image