Provide a Blender to Godot Engine manifold tutorial

[fire]

Your Godot version:

Godot Engine master. godotengine/godot@4364ed6

Issue description:

@tetrapod00 I have a draft of the process for making manifold shapes using Blender's fixing tool. https://docs.google.com/document/d/11KPDwMuZl3c4NSwanhatekAIvUxktURmXJVPkP_a17o/edit?tab=t.0#heading=h.r41iui30hwv7

Let me know if I should work on it more.

[tetrapod00]

In general tutorials like this are not guaranteed to be good things to put into the official docs, since we try to only document Godot.

In this case, I think this is an appropriate addition, because the tutorial is essentially only 3 steps:

1. Install the Blender 3D Print Toolbox addon.
2. Select your mesh.
3. If the mesh is not manifold, press the Make Manifold button.

Changes I would make:

• Screenshots should be a little smaller. If sceenshots of the total window can be around 720px wide without loosing information, they should be. To be clear this means make the window smaller and take a full-resolution screenshot of that 720px wide window.
• In the screenshots where you mention which UI to use, draw a 2px yellow box outline around the buttons meant to be pressed. It's especially needed in this screenshot. I see 5 things in that screenshot which could be "the chevron".
• We also need to explain what "manifold" is. We already mostly do so here: https://docs.godotengine.org/en/stable/tutorials/3d/csg_tools.html#custom-meshes. But we don't use the word "manifold".
• I'm not sure if this tutorial should be a new page or be another section in the existing page Prototyping levels with CSG.
• This tutorial could be as simple as a note block, with no screenshots, that lists the three steps. If the 3D Print Toolbox addon has its own docs we can link to, that might be the appropriate solution.

I think your draft is already basically usable as a starting point. It probably shouldn't get much longer than it is.

You don't need to work on this any more until this is discussed by other docs maintainers.

[fire]

I'll use this extremely technical and precise language for manifold.

Every edge of every triangle must contain the same two vertices (by index) as exactly one other triangle edge, and the start and end vertices must switch places between these two edges. The triangle vertices must appear in clockwise order when viewed from the outside of the Godot Engine manifold mesh.

Which is modified from https://github.com/elalish/manifold/wiki/Manifold-Library#manifoldness-definition

@elalish Is this modification correct?

[tetrapod00]

I think it would make sense to split this into two PRs, actually:

1. Update the existing CSG mesh requirements to mention "manifold". IMO we should include both technical and non-technical explanations of manifoldness. So maybe keep the existing one, and put the technical one after. Linking to the definition used by the library is good to do, too.
2. Add a tutorial for using blender to fix manifold issues, which may need more reviews.

[fire]

I would appreciate assistance on those two pull requests when there is consensus.

[tetrapod00]

As in, help making the PR at all (for RST syntax reason)? If so, sure, I don't mind doing that for either or both.

I think the change to manifold descriptions is pretty uncontroversial so that can be made now and we can discuss changes in the PR.
For the tutorial we should wait for another maintainer to comment maybe

[elalish]

LGTM