Add game state boolean ref docs (#1694)

* Add game state boolean doc refs

* Add check for _img validity

* JS guru tricks

docs/reference/game/is-running.md

@@ -0,0 +1,28 @@
+# is Running
+
+Find out if the game is currently running or not.
+
+```sig
+game.isRunning()
+```
+
+## Returns
+
+* a [boolean](/types/boolean) value that is `true` if the game is running or `false` if not.
+
+## Example
+
+If the game is currently running, end the game if button **B** is pressed.
+
+```blocks
+input.onButtonPressed(Button.B, function () {
+	if (game.isRunning()) {
+        game.gameOver()
+    }
+})
+```
+
+## See also
+
+[is paused](/reference/game/is-paused),
+[is game over](/reference/game/is-game-over)