Add introductory examples in the documentation

[mforets]

The Reachability manual lacks a couple of simple, introductory examples in the documentation that explain the basic use of the library. I am thinking about a simple 1d linear system, and another example with a nonlinear system, for example Lotka-Volterra.

On a second revision we can add some paragraphs giving context of the methods used and the type of questions that can be addressed using Reachability.jl. We can extract this information from our publications.