sysrepo/netconf documentation improvements

Some of yang models have to be installed by hand and some of them are installed automatically as dependencies of those manually installed. So it would be nice that users guide would list every model that have to be installed by hand. I think those models are: ietf-dhcpv6-server.yang kea-dhcp4-server.yang kea-dhcp6-server.yang kea-dhcp-ddns.yang kea-ctrl-agent.yang ietf-inet-types.yang ietf-yang-types.yang for testing: keatest-module.yang

Also docs is missing simple example how to start kea using sysrepocfg, it says Such changes can be done using sysrepocfg tool or remotely using any NETCONF client. For details, please see Sysrepo documentation and I really don't like the fact we are sending user to different documentation instead of having couple lines like:

sudo sysrepocfg -l 4 -d startup -f xml -i startup-4.xml kea-dhcp4-server

sudo sysrepocfg -l 4 -d running -f xml -i twopools-4.xml kea-dhcp4-server

with couple sentences explaining what "running" and "startup" datastores really are.