Add a specification for the PAK format used in Arcturus

[rdw-software]

I started looking into this at some point and I think I had a working implementation, but I forgot and it needs a serious cleanup.

Also, I don't remember if the format is the same as Alpha PAK or if there were any differences.

[rdw-software]

I actually have some notes that look "good enough" to publish, so I'll just add them to the GRF spec and revisit later (...maybe).