Add WebVR section to the documentation

Description of the problem

Right now we don’t have “WebVR contents creation” section in the document. (Let me know if I’m missing.) Then users who wanna create WebVR contents need to pick up necessary info from WebGLRenderer doc and WebVR examples.

I think having WebVR section in the document is very useful to such users. (Actually I had missed I must use renderer.setAnimationLoop so I spent so long time to fix.)

Three.js version

• Dev
• r96
• …

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, …)