Add 3D Particles Documentation

[and-rad]

This adds a new page to the documentation, covering 3D particle systems.

• [x] Emitter properties
• [x] ParticlesMaterial properties
  • [x] Update: sphere surface
• [x] Sub emitters
• [x] Trails
  • [x] Ribbons
  • [x] Tubes
  • [ ] ~Skinned meshes~ (buggy, will be handled in separate PR)
• [x] Collision
  • [x] Box
  • [x] Heightfield
  • [x] SDF
  • [x] Sphere
• [x] Attractors
  • [x] Box
  • [x] Sphere
  • [x] Vector field
• [x] Complex emission shapes
• [x] Turbulence (waiting to be merged and maybe improved)
• [x] Viewport menu
  • [x] Create emission points from node
  • [x] Convert to CPUParticles3D
• [x] Split docs into multiple pages

There is also this companion project that I used to create every screenshot used in the documentation: https://github.com/and-rad/GodotDocParticles

[and-rad]

Small heads up: It might look like I'm not making any progress lately, but that's just me waiting for 4.0 to become a little more feature-complete. I've had to redo some screenshots and gifs multiple times already since I started writing the docs because names and features have changed in the meantime. And with https://github.com/godotengine/godot-proposals/issues/3588 probably getting implemented, I'll have to reorganize a lot of what's already written.

So in an effort to not have to do too many do-overs, I'm keeping the particle docs on the back burner, but I'll pick up the pace again as soon as 4.0 nears RC status.

[clayjohn]

@and-rad Sounds great! Thanks for letting us know. I look forward to when you return to this PR :)

[and-rad]

Gonna link https://github.com/godotengine/godot/issues/61014 here. Even though it doesn't currently work, I have documented the feature as if it did. I expect it'll be fixed in time for the 4.0 release.

[Calinou]

Gonna link godotengine/godot#61014 here. Even though it doesn't currently work, I have documented the feature as if it did. I expect it'll be fixed in time for the 4.0 release.

This wasn't fixed for 4.0's release, so I suggest adding a warning block to mention that the feature won't work yet.

[and-rad]

Done. I deviated from some of your suggestions when I felt it would improve consistency, but I think I got the important parts right.

[mhilbrunner]

Great work! PNGs need to be converted to WebP, but I can do that if you don't want to, seeing how long this been staying open :)

Edit: The PR actually contains WebPs, but PNGs too, we need to get rid of the latter.

[and-rad]

I'll take care of it. I switched to webp halfway through after the decision was made to use the format from now on, but I didn't bother converting the old stuff. I'll replace the gifs with webp too.

[Calinou]

I've amended this PR to document performance caveats of turbulence as per https://github.com/godotengine/godot/issues/76501.