ID 225706 - Update the documentation of the Environment Variables for Mari to include MARI_GIZMO_PATH

Problem summary
The docs do not currently display all of the available environment variables within Mari such as MARI_GIZMO_PATH

Customer reported version

3.1v1

Steps to reproduce

1) Go to the Mari User Guide

2) Search for 'Environment Variables'

3) Check for the presence of MARI_GIZMO_PATH