Add a page documenting gamerules

[Amazinite]

endless-sky/endless-sky#11630

[TheGiraffe3]

I would mention the defaults of each gamerule as well.

[Amazinite]

I would mention the defaults of each gamerule as well.

I originally listed the defaults in the gamerules file because the game could only have one set of gamerules loaded, so I expected some players might edit the file directly in their game directory. Now that players can edit their gamerules within the game without touching the default preset data, or they can make their own preset separate from the default one, figuring out what the defaults are is just a matter of checking the default preset, whether that be in-game or in the data folder.

Duplicating the defaults to here would just mean we'd need to update this page every time we make a change to a default gamerule value.