Party data docs

Mockup of party data.

C++ side is more rough, I wasn't sure if you wanted an example event created for each of the functions or just showing what would go in said functions, as I believe there's at least a couple cases of each in the documentation.

Might need heavy revision. Again, if you think it's something that would require a lot of changes and want to setup a time to voice chat, we can do that.

Edit: Also, haven't added the event descriptions in parties_events.mdx, wanted to know if you wanted a section in that dedicated to party data, or a separate document entirely.