Implement and document Roblox's new Content type

[Dekkonot]

A while back, Roblox renamed all of their Content properties to be a ContentId type. We patched over this in #455 by just using ContentId in lieu of Content for things.

However, there are now properties that utilize the new Content type:

Given that as far as I can tell all of these properties serialize, we should put in the work to actually document this type and implement it.

Unfortunately this means we need to consider the future of rbx_type's Content type. Ideally we would just rename the old Content type to ContentId like Roblox and then add a new Content type but this is a breaking change. :(