Add note on "creating a dynamical system in DynamicalSystems.jl" for high dimensional systems

It came to my attention that some students read our Chapter 1 and the use the very first code snippet to make their own dynamical system. That is great if the system is sufficiently low dimensional. However, some students use the same code to make any dynamical system, such as e.g., a coupled oscillator network which could be arbitrarily high dimensional.

We should have a clear note in that section warning about this. Something like:

Note that Code 1.1 is meant to be used when the dynamical system is low dimensional. For high dimensional systems an alternative code should be used that is described in the documentation of DynamicalSystems.jl

Open Chapter 1 and locate the first dynamical-system code snippet, identified as Code 1.1. Add a clear note there that it is intended for low-dimensional systems and that high-dimensional systems should use the alternative approach described in the DynamicalSystems.jl documentation.