Add initial modding API docs

docs/godot/cppapi.md

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 # C++ API Reference

docs/godot/modding.md

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+
+# Modding and APIs
+
+## Modding
+
+The most basic way to add modding support to a game using Godot Sandbox is to add a Sandbox somewhere, and load a program into it.
+
+```py
+func activate():
+    var reader = ZIPReader.new()
+    reader.open("res://scenes/lua/lua.zip")
+    var buffer = reader.read_file("lua.elf")
+    var s : Sandbox = get_node("MyVM")
+    s.load_buffer(buffer)
+```
+
+With the program loaded, the program in the Sandbox will go through an initialization phase, and could possibly perform every action that it needs to then and there. In theory, no modding API is needed and instead just a general map of the scene trees in the game is enough.
+
+## A simple modding API
+
+A modding API is there to make it easier for modders to interact with your game. Modding APIs should direct modders towards useful features and mechanisms in your game.
+
+```py
+	var api : Dictionary
+	api["get_player"] = func(): get_node("Player")
+	...
+
+	s.vmcall("set_api", api)
+```
+
+Passing the modding API as a dictionary to a Sandbox function is quite OK as a modding API. It should be documented somewhere else.