Document GPU-accelerated ParaViewWeb rendering using VirtualGL and TurboVNC

We have researched a method for using VirtualGL and TurboVNC in order to
provide hardware-accelerated rendering capability to ParaViewWeb, even in
situations where there is not a physical display. We need some
documentation of the steps required in order to reproduce this result on
the ParaViewWeb documentation site.

Change-Id: I471e8f077c06963da483d6417c1ea3231f3e89b7
Merge topic 'pvweb-virtualgl-doc' into next

db5295c Document GPU-accelerated ParaViewWeb rendering using VirtualGL and TurboVNC