z_demo documentation

[fig02]

documents z_demo.c and related commands. Paves the way for documentation of the cutscene actors (which should probably all be documented at once since they are all very similar)

Some notes:

• There are many changes needed zap-side after this is merged to use the updated macros and enums.
• The descriptions of the command macros themselves could probably use some work.
• The names I gave to things in the cutscene actors are not meant to be taken as fact, only a suggestion. They can be improved when the cutscene actors get documented. What I aimed for is consistency among the names, so they can all be easily found and replaced when needed.