Added documentation

[danielbekhter]

Added XML code documentation as well as markdown articles.

[BenediktHensen]

I added cross-references using the xref:uid links. Moreover, each sentence should be in its own line so that we can track changes to the documentation easier. I also updated the headlines so that they follow proper APA capitalization standards.

There was also something strange going on with the API part of the documentation and it failed to generate. I updated the settings and now we get the full API documentation, too. Unity also generates an error because the package references itself in the Unity project, so I removed this circular dependency.

With these changes, the pull request can be merged.