Added structure for OPNFV documentation

Having this structure in place is a requirement for passing release
milestone M2. This change does not modify the content of the
current documentation / deployment guide.

Signed-off-by: Georg Kunz <georg.kunz@ericsson.com>
Change-Id: I07e8820901970da1199f4a474161e49f25707ecc
Reviewed-on: https://gerrit.opnfv.org/gerrit/c/kuberef/+/71298
Tested-by: jenkins-ci <jenkins-opnfv-ci@opnfv.org>
Reviewed-by: Michael Pedersen <michaelx.pedersen@intel.com>
Reviewed-by: Sofia Wallin <sofia.wallin@est.tech>