Add documentation

- Add file documentation to about 1/4 of the files in Source
- Copy over a lot of the documentation from the sanctuary/notes repo
- Standardise all the existing documentation
- Create a configuration for Doxygen
- Add more documentation (engine.cpp is now fully documented)

Source/encrypt.cpp

@@ -1,3 +1,8 @@
+/**
+ * @file encrypt.cpp
+ *
+ * Implementation of functions for compression and decompressing MPQ data.
+ */
 #include "all.h"
 #include "../3rdParty/PKWare/pkware.h"