Update some of the 3D particles pages for 4.2

[skyace65]

Updates the particles pages with information primarily from this blogpost and the class reference page. This isn't everything from 4.2, most of the new process materials properties are still not documented however I've re-organized the page so everything is in the same order as what's in the 4.2 editor, updating it in the future should be easier now. Here is a list of all changes:

• Mentions the new limitations of CPU particles
• Mentions that you can now convert from CPU particles to GPU particles
• Documents the new GPUParticles nodes properties
• Updates 2 images on the GPUParticles node page
• Documents the new damping as friction property
• Re-orders the process materials page

I have no plans to add info for the other new properties in this PR, it can be merged as is after review.

[skyace65]

Fixed.

[mhilbrunner]

Thanks! Merged.