doc: split the "doc" target into setup and building
doc: split the "doc" target into setup and building

Other targets need the setup step but not the building, so split
up the target to avoid unnecessary work.

Signed-off-by: Ruth Fuchss <90c286bb8b7c8291b9f5a2a1d38dc233728381fe@canonical.com>

lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd