Add doc section for ROS2 users

[markaren]

threepp itself is not tied in any way to ROS2, however, it fits very nicely as a visualiser in ROS2 nodes.

• It is easy and quick to integrate
• It supports URDF and can handle share packages
• Easy to visualise points clouds.
• Can emulate LIDAR and RGB-D sensors

TLDR; Some instructions on how to integrate should be added.

[markaren]

With #393 users may now use ROS2 directly in Python scripts within the editor (this assumes a sourced shell naturally). The possibilities are endless.