Clean up the "Getting Started" docs

[ChuckJonas]

The "getting started" docs are incredibly confusing. Instead of introducing a simple "hello world" program to get a script up and running, there seem to be multiple concepts and parallel paths being introduced simultaneously. Instructions should be explicit and comprehensive.

[pelikhan]

I gave a shot at splitting down the getting started steps. https://microsoft.github.io/genaiscript/getting-started/