Follow naming convention of textures and sounds in doors

mods/doors/init.lua

mods/doors/sounds/doors_door_close.ogg

mods/doors/sounds/doors_door_open.ogg

mods/doors/textures/doors_brown.png

mods/doors/textures/doors_glass.png

mods/doors/textures/doors_glass_a.png

mods/doors/textures/doors_glass_b.png

mods/doors/textures/doors_glass_side.png

mods/doors/textures/doors_grey.png

mods/doors/textures/doors_obsidian_glass.png

mods/doors/textures/doors_obsidian_glass_a.png

mods/doors/textures/doors_obsidian_glass_b.png

mods/doors/textures/doors_obsidian_glass_side.png

mods/doors/textures/doors_steel.png

mods/doors/textures/doors_steel_a.png

mods/doors/textures/doors_steel_b.png

mods/doors/textures/doors_trapdoor.png

mods/doors/textures/doors_trapdoor_side.png

mods/doors/textures/doors_wood.png

mods/doors/textures/doors_wood_a.png