Update links to ROS 2 documentation. (#179)

I am not sure if these index links used to work previously but they
aren't valid currently.

I had started down the road of updating the tutorial to current ROS 2
distributions when I realized that this tutorial might be restricted to
Gazebo Classic and I'm not sure how much of an overhaul it might need
for Humble and Iron.

Happy to continue updates in follow-up PRs if it makes sense to do so.

Author: nuclearsandwich

ros2_installing/tutorial.md

@@ -12,12 +12,12 @@ debian packages or from source.
 ## Prerequisites
 
 You should understand the basic concepts of ROS 2 and have gone through some
-[ROS 2 Tutorials](https://index.ros.org/doc/ros2/Tutorials).
+[ROS 2 Tutorials](https://docs.ros.org/en/foxy/Tutorials.html).
 
 ### Install ROS 2
 
 ROS2 can be installed either through binary installation or source installation,
-see the [ROS 2 installation page](https://index.ros.org/doc/ros2/Installation).
+see the [ROS 2 installation page](https://docs.ros.org/en/foxy/Installation.html).
 The current stable distribution is **Foxy**.
 
 > **Tip**: Don't forget to source `setup.bash` script as instructed