Add ROS 2 and real-time introductory document

[carlossvg]

The goal is to create a document that serves as a starting point for users interested in ROS 2 and real-time. This document would cover:

• What real-time really means. Very fast” versus “deterministic”.
• What is required to have ROS 2 real-time capable applications
  • Design principles of the ROS 2 real-time system.
  • What software and hardware to select. Safety aspects consideration.
  • Programming Language choice.
  • Real-time operating system overview.
  • ROS 2 RTOS support.
• Current status and future work in ROS 2

We can use this document as a reference: http://design.ros2.org/articles/realtime_background.html. We shouldn't overlap the scope of this document with the concepts section or the best practices guide. Those topics should be covered in separate documents.

Acceptance Criteria

• [ ] Add ROS 2 and real-time introductory document

[carlossvg]

Closed in favour of https://github.com/ros-realtime/rtwg_documentation/issues/16