Raytopia

As per my PR #131 I have adjusted the Introduction to Panda3D section so it flows better and is closer to the community values set forth on the [get-involved ](https://www.panda3d.org/get-involved/)...

I think that this would be good for a cheat sheet. I can get started on this if no one else wants to.

This should be easy to implement once #1643 is merged (I need to finish up my changes though). The example GLSL is only 13 lines so super easy to add....

Yes it is optional. Re-reading their specification it's for reference. I'll get it implemented for 1.10.15