Rearange ROS 2 tutorials and ROS/Gazebo install

This issue comes from here https://github.com/gazebosim/ros_gz/issues/708
We discussed reorganizing ROS 2 tutorials inside the Tutorials section. ROS/Gazebo installation would also be included in this tutorials. There are two problems to discuss:

- It seems that multi-level is not possible at the moment (see https://github.com/gazebosim/docs/blob/a574980d6aa853ac52af0a03f02f2e08d345fa89/build_multiversion.py#L127) so I added it alongside the Tutorials, which will also be more visible to ROS users. Should "ROS 2 tutorials" be on the top level or within Tutorials? The second would require implementing recursive children levels.

- ROS/Gazebo install is applied to all the docs, so it cannot be moved to the ROS 2 Tutorials as a subsection unless it's done for all the versions.