How can I make mods?

[typo255]

There is a folder in %ColobotGoldInstallDir%\data (or $ColobotGoldInstallDir/data for *nix users) called "mods". It is clear that this folder is for user-added content, but it is unclear:

1. What kind of content can I put into this folder? Is it limited to userlevels, or can I put music, models, maps, readily-available code to add to the CBOT API, and/or documentation into it?
   
2. How must I structure the content's folder for Colobot Gold to recognize the content?
   
3. I searched the old Colobot website. The old link to "Creating Colobot user levels" is dead. It makes sense that Colobot Gold would mod differently than the old Colobot, but it would be similar. How can I make a userlevel?
   

If there are one or more thread(s) in this forum that answer these questions, could we arrange to sticky them or otherwise make them easier to find? Before I posted this thread, I searched "mod" and 'mods" and found nothing more useful than custom user levels. I could reverse engineer the available userlevels, but I would rather have straightforward documentation, and I am certain I am not the only one. If there is no such documentation, I will do the work necessary to document how mods work and put it on Github during my free time unless I find someone who has done it before me.