minetest_game/nether

For a description of this Minetest mod, please refer to the forum topic: https://forum.minetest.net/viewtopic.php?f=9&t=10265

TODO:

• Find a way to get the perlin noise inside [-1; 1] or use another noise
• Add some node which containing items to the simple pyramid "buildings"
• Change the sky background, fog, etc. in the nether
• Improve the guide formspec. Writing it in Markdown would be nice. These could be useful:
  • https://forum.minetest.net/viewtopic.php?t=23855 Minetest Bedrock Markup Language
  • https://github.com/mpeterv/markdown Markdown to HTML converter; Minetest formspecs support HTML