Dedicated doc section to explain all ROS 2 environment variable

[fujitatomoya]

Overview

Current document already has explanation and information about ROS 2 environment variables in each related section. This is okay but not well-structured for the user wants to know about ROS 2 environment variables, since the information is dispersed. It would be probably better to have dedicated section to explain all ROS 2 environment variables in one place, and that should be referred from each related section as needed.

Related Issue

• https://github.com/ros2/ros2_documentation/pull/3014#pullrequestreview-1125965720