Basic documentation and developer guides

[mortendahl]

We are getting to a point where the design of the new framework is stabilising (yet mostly undocumented) and where more people are potentially joining and development of both the runtime and the compiler. To that end it will be beneficial to increase documentation in the code, as well as start writing some basic architecture documents; this will help not only new people but also make sure core developers are aligned.

[mortendahl]

Quick idea for how to think about dialects/levels (some inaccuracies, bare with me)