Rework the script; update README

[jhanca-robotecai]

Based on the previous works I refactored the python script to follow the structure of the agreed scenario with three segments recorded in different simulators.

Section 1 (O3DE) : 90 sec
- Segment 1.1: Open rclpy script code that starts with code, load our shared world (conditionally).
- Segment 1.2: Spawning an object and despawning after a delay.
- Segment 1.4: Update code to spawn robots (Dingo and UR10e) and some static obstacles. Run script and visualize in a simulator.
Section 2 (Gazebo) 90 sec
- Segment 2.1: Add code for Play / Pause.
- Segment 2.3: Add code for setting state of objects (box)  and showcase.
- Segment 2.2: Add code for stepping simulator for some frames. Show the simulator running for a number of frames.
Section 3: (Isaac Sim) 90 sec
- Segment 3.1: Add simple loop code that sends target goals and queries entity states.
- Segment 3.2: Next show code that “randomizes” the scene with boxes or something once robot had reached a certain pose.
- Segment 3.3: Next run full scenario.

I tried to keep it as similar as possible to the previous implementation (in #2), but some differences can be noticed.

The script was primarily refactored to:

1. Split the part that is different for every simulator
2. Make simple tutorial-like helper methods to call certain simulation interfaces
3. Move the extras (such as moving UR) to a separate file to keep the content as simple as possible

I also included some small changes in the README file, that should be completely refactored in a separate PR.

This script was tested in O3DE only.

@ayushgnv @iche033 @adamdbrw
Please feel free to comment on any parts that still need some work.

[jhanca-robotecai]

Thank you for your review.
Together with @adamdbrw we applied and tested the changes in O3DE (using ROS 2 Humble and ROS 2 Jazzy). Feel free to reopen any of the conversations if you find some missing changes.

[adamdbrw]

@ayushgnv
@iche033
To be merged on your approval