Write a page about 3d convention conversion

**Your Godot version:**
https://github.com/godotengine/godot/commit/0c23a2cfe3ad897e1e49008629c135764b2c155c

**Issue description:**

Document the 3d conventions problem between different 3d software:

1. Axis convention https://twitter.com/FreyaHolmer/status/1325556229410861056?s=20&t=jFYr1IodhxYSod0JnhpcbQ


3. Quaternion convention https://github.com/godotengine/godot-docs/issues/6271
4. Normal maps https://bgolus.medium.com/generating-perfect-normal-maps-for-unity-f929e673fc57
5. 6 unique ways to convert euler to quaternion https://en.wikipedia.org/wiki/Conversion_between_quaternions_and_Euler_angles
6. Roughness metallic vs glossiness specular https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_specular/README.md
7. Per mesh skeleton skinning vs one skeleton armature https://github.com/KhronosGroup/glTF-Blender-IO/issues/1652
8. Projection (Transform 4x4), Godot Transform3D (3x4), Basis (3x3 mat) vs euler (vector3) vs axis angle (vector3 and floating point) vs quaternion (vector 4)
9. 3d rotation error in representations with a visual diagram. https://zhouyisjtu.github.io/project_rotation/rotation.html
11. matrixes stored in row major or column major
10. C++ / core code uses radians. The inspector uses degrees.
11. GLTF2's cubic keyframe interpolation is not the standard cubic interpolation in 3d editing software
12. Not really 3d. But 2d has a different axis convention..
13. global pose minus rest vs Global pose animation. Change from godot 3 to godot 4.
14. Godot uses meters, but some things use other units, like Unreal uses centimeters.

**URL to the documentation page (if already existing):**
Not sure.