In case you are not so much interested in programming for Adonthell but rather in creating game content you should take a look at the Designer's documentation instead.
The Introduction for new programmers gives an overview about the engine's foundamentals. Programming languages, source code organization and such. Before you dive into the code, have a look at this for the basics.
Of similar importance -- if you want to lend a hand to the coding -- are the Programming rules . They include indentation rules, naming conventions and a few other odds and ends that shall help to give the code of different programmers a similar shape to make it easier to read.
Once you understand the basics, you might want to explore specific parts of the engine in more detail. As many parts of the game are divided into several classes, these chapters are there to give a first overview how they work and interact.