add(ui): add intro scene #55

...hdm_stuttgart/battlearena/Controller/IntroController.java

...gart/battlearena/Controller/MenuBorderPaneController.java

src/main/java/de/hdm_stuttgart/battlearena/Main/Main.java

src/main/resources/fxml/Intro.fxml

src/main/resources/fxml/MenuBorderPane.fxml

src/main/resources/styles/style.css

src/main/resources/textures/videos/monke.mp4

src/main/resources/textures/videos/pepe.mp4