Add libraries docs

Author: fwsGonzo

docs/godot_docs/cppapi.md

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 # C++ API Reference

docs/godot_docs/libraries.md

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+
+# Program Libraries
+
+Libraries make it easy to use pre-made programs in the sandbox.
+
+```py
+    if !FileAccess.file_exists("res://luajit.elf"):
+        var buffer = Sandbox.download_program("luajit")
+        fa = FileAccess.open("res://luajit.elf", FileAccess.WRITE)
+        fa.store_buffer(buffer)
+        fa.close()
+
+    var luajit = Node.new()
+    luajit.set_script(load("res://luajit.elf"))
+
+    luajit.add_function("test", func(name): return "Test " + str(name) + " called!")
+    luajit.add_function("add", func(a, b): return a + b)
+
+    luajit.run("""
+    print(test(1))
+    print(add(333, 666))
+    function fib(n, acc, prev)
+        if (n < 1) then
+            return acc
+        else
+            return fib(n - 1, prev + acc, acc)
+        end
+    end
+    print("The 500th fibonacci number is " .. fib(500, 0, 1))
+    """)
+```
+
+## Godot Sandbox Programs
+
+There is [a repository with pre-made programs](https://github.com/libriscv/godot-sandbox-programs).
+
+You can find [the latest release here](https://github.com/libriscv/godot-sandbox-programs/releases/latest). The latest release is used by `Sandbox.download_program(name)`.