add doc/**/cameras.md & update cameras library

doc/en/scripting/builtins/libcameras.md

@@ -0,0 +1,66 @@
+# *cameras* library
+
+Cameras manipulation library.
+
+## Main functions
+
+```lua
+cameras.get(name: str) -> int
+```
+
+Returns a camera by name.
+
+## Camera methods
+
+```lua
+-- accessing camera
+local cam = cameras.get("pack:camera-name")
+
+-- returns camera index
+cam:get_index() -> int
+
+-- returns camera name
+cam:get_name() -> str
+
+-- returns camera position
+cam:get_pos() -> vec3
+-- sets camera position
+cam:set_pos(pos:vec3)
+
+-- returns camera rotation
+cam:get_rot() -> mat4
+-- sets camera rotation
+cam:set_rot(rot:mat4)
+
+-- returns camera zoom value
+cam:get_zoom() -> number
+-- sets camera zoom value
+cam:set_zoom(zoom: number)
+
+-- returns angle of the camera's field of view in Y (degrees)
+cam:get_fov() -> number
+-- sets camera field of view angle in Y (degrees)
+cam:set_fov(fov: number)
+
+-- returns true if the Y axis is flipped
+cam:is_flipped() -> bool
+-- reflects the Y axis when true
+cam:set_flipped(flipped: bool)
+
+-- checks if perspective mode is enabled
+cam:is_perspective() -> bool
+-- turns on/off perspective mode
+cam:set_perspective(perspective: bool)
+
+-- returns camera direction vector
+cam:get_front() -> vec3
+-- returns camera right vector
+cam:get_right() -> vec3
+-- returns camera up vector
+cam:get_up() -> vec3
+
+-- makes camera look to a given point
+cam:look_at(point:vec3)
+```
+
+Use player.set_camera to switch cameras.