Plugins tutorial should be improved to be more new user friendly

[azeey]

          * **Test enviornment**
```text
CPU: Ryzen 5 5600x, Ram: 32GB @ 3200 Mhz, GPU: RTX 3080
Clean copy of Ubuntu 24.04 Humble
ROS 2 Jazzy Jalisco full desktop
```

• Summary
  • In my humble opinion this tutorial may need to be presented in context of a more hands-on example. The current version was hard to follow, specially as a newcomer to Gazebo.
  • Add a note stating if built from source, GZ_GUI_PLUGIN_PATH can be set as follows

  cd ~/workspace/src/gz-gui
  export GZ_GUI_PLUGIN_PATH=${HOME}/workspace/src/gz-gui
  echo $GZ_GUI_PLUGIN_PATH
  gz gui -s ImageDisplay

  • Tutorial does not show how to set GZ_GUI_PLUGIN_PATH. Nor it states the user had to change direcotry into src/gz-gui directory before this command would work correctly

• Issues
  • Users needs to change directory to ~/workspace/src/gz-gui. Without this step, invoking any of example commands with gz gui -c prefix throws XMLError error.
  • The tutorial page has multiple \subpage references that leads to nowhere.
  • Commands for Topic Viewer, Topic Stats, Requesters and Responders did not work.

Originally posted by @Mechazo11 in gazebosim/gazebo_test_cases#1323 (comment)