New nether guide

* Remove code which uses the font size setting to position text and images. Instead use Minetest's hypertext formspec element
* Rewrite many parts of the guide contents, e.g. "you" -> "we", more detailed portal description
* Add a setting to permit viewing it in the overworld (enabled by default)
* Update the Readme

I have set the font size to 24, which works well for me on a 1920x1080 screen with somewhat high DPI. The text written in hypertext apparently ignores the Minetest font size setting, so it may look bad on different screens.

nether/settings.lua

@@ -2,6 +2,7 @@ local default_settings = {
 	trap_players = true,
 	log_to_chat = false,
 	log_level = 2,
+	overworld_help = true,
 }
 
 nether.settings = {}