Resolve "Make the user guide user friendly"

@erhart carried out the changes suggested in #364 (closed) and I added a bit more info in the background section plus a page describing how to cite icet now that the paper is out.

The additions to the background section is really minimal, partly because I couldn't muster up the energy to write a lot of theory (that probably no-one will read). For the theory I think it makes sense to refer people to some papers primarily instead, so I did that on top of the Cluster expansions > Formalism page. If the icet community considers this sufficient, we could also close #176 (closed).

Closes #364 (closed)