Add documentation on "3D" videos

Multiple points here to be described separately, and explain what is supported in VLC. We probably want to show detailed samples of what format it is with and without VLC processing.

There is no control in VLC 3.0 to force a given projection/stereo mode so adding information about how to inject the required metadata for mp4/mkv/jpeg is probably a good idea. Troubleshooting with mediainfo/mediainfo-gui might give advanced users a hint on what to report/to do.