Generate SDK Documentation with Doxygen Instead of Headerdoc

[njsch]

Currently it only seems possible to test Headerdoc-generated content on Mac and possibly Linux. Windows could be included in these checks if Doxygen was used instead.

Additionally, I'm also not sure whether Headerdoc may even be one of Apple's legacy projects. I was wondering if it would be more efficient to have something more universally cross-platform and potentially not deprecated.

It does not appear that it would require too much work in terms of modifications to dev docs found in the SK-core code. But I haven't really looked into this much yet, it's just a thought that occurred when three out of four checks failed on some code I have written.