Provide a VR home page

[ric-bianchi]

Currently, the choice of which experiment the user wants to explore is done in a standard HTML page. From there, links open the ATLAS and CMS pages, which can be switched to VR mode when looking at them with VR goggles.

The problem is that, while being on an experiment's page in VR mode, if users click on the "back" button in the VR GUI, they are pushed out from the VR mode because the standard home web page is displayed. While using VR goggles like the Oculus Go, that means that VR mode of the browser is closed and the standard Oculus "living room" is shown to the users, making them leave the virtual world.

So, we should provide a landing page with VR implemented in there already, so the VR mode is kept when switching from one experiment to another one.