Getting started doc rewrite

[ArnoStrouwen]

The getting started documentation is currently a confusing combination of sometimes using the mtkmodel macro, and sometimes not. This PR puts as much functionality as possible into said macro, and removes all sidetracking explanations of the more manual definitions of ODESystems and ODEProblem.

I'll also take the opportunity to note the sentences in the tutorial I did not fully understand:

And the independent variable t is automatically added by @mtkmodel.

Added where exactly?

Vector-valued parameters and variables are possible. A cleaner, more
consistent treatment of these is still a work in progress, however. Once finished,
this introductory tutorial will also cover this feature.

Is this ready by now?

Observed equations are variables which can be computed on-demand but are not necessary for the solution of the system.

Calculating RHS is definitely needed for solving the system (you can see this by inspecting prob.f.f.f_oop, it is just not a state of the dynamic system? I don't know how to succinctly rewrite this.