Header footer guidance

This is very much a work in progress. The language probably sucks, there are no images yet and the templates stuff is currently pie in the sky.

I am thinking that the templates & css will work similarly to how the header-footer-only templates & css also work. I envisage creating a separate template file for each of the combinations listed in the guidance. 

@edds @phae and @antimega probably have views/better ideas on this.