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

[JanStaschulat]

update https://github.com/ros2/design/edit/gh-pages/articles/120_realtime_background.md

create a PR on create a fork on rtwg : https://github.com/ros2/design/edit/gh-pages/articles/120_realtime_background.md