Added Sphinx based API docs. I am still a bit unsure if and how I should add the generated files to the repository. I read a bit about how to integrate this with the GitHub gh_pages branch but I am still pondering. For now you need sphinx for Python 3 and can call "make html" in documentation/src/.

Writing the documentation has the benefit of showing clearly were the architecture is not clean yet. There is the code generating the latch for the FlexBoxes which is still done with methods although a lot of this looks pretty much like an Edge. So this needs to be converted into Edge classes at some point.

I also added a new Edge class: StackableEdge that adds feet to the corners of boxes and recesses in the top corners so they can fit on top of each other without sliding around. I also added parameters for Box2 to change the top and bottom edges. That's the only way of using the new edge type with the existing generators for now. Not being able to actually cut the boxes out and testing how well they handle is pretty frustrating, though.

I plan to add more stuff to Box2 - may be an option for a treasure chest like lid and may be also a flat lid. May be I need to rename it to Box and move the current Box further back in the list as it is much less useful.