add gfx.skeletons library docs

doc/en/scripting/builtins/libgfx-skeletons.md

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+# gfx.skeletons library
+
+A library for working with named skeletons, such as 'hand',
+used to control the hand and the carried item displayed in first-person view. 
+The set of functions is similar to the skeleton component of entities.
+
+The first argument to the function is the name of the skeleton.
+
+```lua
+-- Returns an object wrapper over the skeleton
+local skeleton = gfx.skeletons.get(name: str)
+
+-- Returns the index of the bone by name or nil
+skeleton:index(name: str) -> int
+
+-- Returns the name of the model assigned to the bone with the specified index
+skeleton:get_model(index: int) -> str
+
+-- Reassigns the model of the bone with the specified index
+-- Resets to the original if you do not specify a name
+skeleton:set_model(index: int, name: str)
+
+-- Returns the transformation matrix of the bone with the specified index
+skeleton:get_matrix(index: int) -> mat4
+
+-- Sets the transformation matrix of the bone with the specified index
+skeleton:set_matrix(index: int, matrix: mat4)
+
+-- Returns the texture by key (dynamically assigned textures - '$name')
+skeleton:get_texture(key: str) -> str
+
+-- Assigns a texture by key
+skeleton:set_texture(key: str, value: str)
+
+-- Checks the visibility status of a bone by index
+-- or the entire skeleton if index is not specified
+skeleton:is_visible([optional] index: int) -> bool
+
+-- Sets the visibility status of a bone by index
+-- or the entire skeleton if index is not specified
+skeleton:set_visible([optional] index: int, status: bool)
+
+-- Returns the color of the entity
+skeleton:get_color() -> vec3
+
+-- Sets the color of the entity
+skeleton:set_color(color: vec3)
+```