[Documentation] Address #2850: Mention GZ_SIM_SYSTEM_PLUGIN_PATH (#2863)

Signed-off-by: Arjo Chakravarty <[email protected]>
(cherry picked from commit db31641)

Author: arjo129

tutorials/create_system_plugins.md

@@ -114,3 +114,6 @@ In the SDF file representing your simulation, add the plugin to the `world` sect
     </plugin>
     ...
 ```
+Plugins can be found using the `GZ_SIM_SYSTEM_PLUGIN_PATH` environment variable. If you
+are using colcon to build your package consider using hooks to add your system to
+`GZ_SIM_SYSTEM_PLUGIN_PATH`.