What coordinate system does Open3D use?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I can't find any reference to the coordinate system used in Open3D.

For instance, OpenGL uses RH y-up (z backward), Blender uses RH z-up, DirectX uses LH y UP, and Unreal uses RH with x to the right and y up).

```
# OpenGL RH y UP
# y
# |
# +---x
# /
# z

# Blender RH z UP
# z
# | y
# |/
# +---x

# Direct X (not used here), LH y UP
# y
# | z
# |/
# +---x

# Unreal Engine, RH, x RIGHT y UP
# y
# |
# +---x
# /
# z
```

This is pretty fundamental, so I probably just missed it in the docs. If it's not in the docs, this would be really helpful to add up front in the docs.

I'm passing my data (point cloud, meshes) through several applications with potentially different coordinate systems, and want to make sure I have all my transformations correct. I'm quite familiar with this domain, so I only a reference to which coordinate system is in use. Thanks!