add 'assets' library docs

doc/en/scripting/builtins/libassets.md

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+# *assets* library
+
+A library for working with audio/visual assets.
+
+## Functions
+
+```lua
+-- Loads a texture
+assets.load_texture(
+    -- Array of bytes of an image file
+    data: table | Bytearray,
+    -- Texture name after loading
+    name: str,
+    -- Image file format (only png is supported)
+    [optional]
+    format: str = "png"
+)
+
+-- Parses and loads a 3D model
+assets.parse_model(
+    -- Model file format (xml / vcm)
+    format: str,
+    -- Contents of the model file
+    content: str,
+    -- Model name after loading
+    name: str
+)
+```