Move Doc struct to Doc.h

[JosiahWI]

This splits out the Doc struct to its own header. The reason for doing this is to use the struct in AggregateWriteBuffer.h without introducing a direct circular dependency on P_CacheVol.h, but it seems good even as a standalone change so I'm PRing it separately. I moved some #defines that went along with the Doc struct as well.

The second commit in this PR adds a self_type alias to Doc. This might not have been necessary; I was looking for various things to clean up and did it while I was messing around.

[JosiahWI]

I heard some concerns from @cmcfarlen about putting moving this to a "public" header when it was in a "private" one before.