Template docs

https://github.com/sonata-project/SonataAdminBundle/issues/1531

Also, I've removed some stuff from the configuration section, as part of https://github.com/sonata-project/SonataAdminBundle/issues/1522. The parts if removed are the ones already covered on revised docs pages. This way, we know what's missing. Also, I've left a warning telling users what's happening to the configuration page