[Steam] Add SteamGridDB assets

[elf-alchemist]

SteamGridDB, or SGDB, has recently started the Grid and Tear project, to as many custom Steam Library assets to as many Doom WADs (and Doom related software) as possible. I requested the SGDB team that Eureka be added as well.

I also ask for permission,that I may add the current logos and icons for this effort, or make my own assets derived from these.

Eureka's SGDB page

[elf-alchemist]

For reference both SLADE and UDB already have their own pages, too.

SLADE UDB