Rewrite "Importing 3D scenes" page

[FreshlyBrewedCode]

Your Godot version: 4.0 beta1

Issue description: The current documentation about importing 3D assets from external tools such as Blender is outdated. It does not cover the new workflow present in the latest 4.0 beta release. It also seems to be missing some notable 3.x features.

URL to the documentation page (if already existing): Importing 3D scenes

Context

I've recently started experimenting with Godot and was struggling to find a good workflow for importing 3D assets. This was partially because the latest documentation is not up-to-date with the latest beta build of Godot 4.0. However, I found it generally difficult to figure out a reliable 3D workflow from Blender to Godot using just the official docs.

Missing/Outdated Documentation

Related to #5121.

• Document new 4.0 import workflows
  • Presets do not seem to exist anymore instead the advanced import dialog has to be used to extract meshes and animations or use external materials
• Document the .blend import feature (https://github.com/godotengine/godot/pull/54886)
  • Mention related editor settings:
  • Also relevant for 3.x?
• Possibly mention the limitation/bug of imported scene instances not getting updated and possible workarounds (if godotengine/godot/pull/57606 does not get merged in the meantime).
• Add more information about how animations are imported
  • AnimationPlayer is created for an imported scene
  • Recommended workflow
  • Animation retargeting (Godot 4.0) (https://github.com/godotengine/godot-proposals/issues/4510)

Improvements

Here are some general ideas for improving the clarity of the page:

• Instead of having several Exporting \ from Blender sections, create sections for different formats/tools (e.g. "Importing from Blender", "Importing from Maya", "Importing FBX").
  • Cover the available options for getting 3D assets from a given tool into Godot and mention the recommended approach (if applicable)
    • Blender
    • Maya
    • 3DS Max
    • Substance
  • Cover specific limitations/features/considerations for a given file format
    • glTF
    • FBX
    • OBJ
    • DAE

Please note that these are points I noticed after only several days of using Godot. I may have missed documentation that is already available elsewhere. However, I assume my experience is similar to what many other first time Godot users experience, and this is a common topic for first time users.

[yume-tea]

Just wanted to add that this section on custom import scripts is also outdated. The post_import(scene) method was changed to _post_import(scene) in Godot 4

[Deozaan]

I found what I believe is a copy-paste error on this documentation page.

Generate > NavMesh: If checked, generates an OccluderInstance3D sibling node for occlusion culling using the mesh's geometry as a basis for the occluder's shape.

I believe it should say Generate > Occluder or something similar instead of NavMesh. There is a paragraph just above it that is about NavMeshes.