Catalyst introduction doc update

[TorkelE]

updates the catalyst intro doc

[TorkelE]

The plan is to basically split this one. This one then because a pure intro to Catalyst that do not really assume that the user have read anything else (but are familiar with basic Julai syntax). Next, we have a "Advanced introduction to Catalyst" or "Introduction to Catalyst and its internals", or something (not really sure of its name) where we go through stuff like how system conversions work, symbolic representations, and give examples of the various types of modelling approaches (DSL, Programmatic, Compositional).