Add documentation page

[Ratsch0k]

Add some kind of documentation page for this project.

It should contain the API and other information about the project.
We can maybe also use it to document some information about the game's internal workings.

Currently considered are two different options.
Either use GitHub's wiki function or create a full documentation webpage using GitHub pages.
While the latter is more work, it would be significantly easier to customize to our liking.