Bring "Why it works" closer to the modelled generative process

[SamuelBrand1]

We discussed this in #91 but I'm starting to lean more towards rewriting the "Why it works" description so that it precisely does the maths underlying the generative model described in "Getting started".

The reason why "Why it works" approaches the problem from the perspective it takes is essentially subjective, and it would probably be better to bring the maths towards the code.

Of course, noting that that the approaches are all mathematically equivalent but I think this would make it clearer for users.

[seabbs]

We now have the analytical solutions written from the perspective of the why it works so if doing that we would need to update those as well

My preference is to keep it coming from the direction it does but to more clearly link out to the expressions elsewhere in the docs.