fix: documentation images and links

[rafa-martin]

This pull request significantly restructures and updates the README.md to improve clarity, organization, and usability for users working with Robotnik's ROS 2 simulation packages. The new README provides a clearer quick start, better separation of setup instructions, and improved documentation for custom simulation and Docker usage.

Key improvements and changes:

Documentation structure and clarity:

• Reorganized the README with a new table of contents, clearer sectioning, and improved headings for easier navigation and readability.
• Updated badges, shields, and logo presentation for a more professional appearance.
• Added a summary "About" section and a dedicated "Demo" section with links and images.

Installation and setup instructions:

• Consolidated and clarified the prerequisites, installation, and build steps, including explicit commands for setting up the workspace and installing dependencies.
• Provided new instructions for installing Robotnik and related packages using vcs import and prebuilt debs.

Usage and customization:

• Added explicit usage instructions for launching worlds, spawning robots, and controlling them, with clear code examples.
• Expanded and clarified sections on Docker usage and environment setup. (F6fe227fL1

[robert-ros]

Great job porting changes from humble! Here are a few points to consider:

robotnik_simulation/README.md

Consider include the installation instructions in the main README to increase visibility and impact, at least until the Docker-based setup is fully available.

robotnik_simulation/robotnik_gazebo_ignition/README.md

README should first provide the installation instructions and then explain how to launch the simulation.

[rafa-martin]

Thanks for the feedback.

• Main README: Installation depends on the selected simulation. The main README will be the entry point. It will list available simulators and their compatibility, and link to per-simulation install instructions.

• robotnik_gazebo_ignition README: I prefer to show usage first. Those steps are more stable and better reviewed, and they let users see results sooner. I will still add a brief note and link reminding that installation must be completed before execution.

[rafa-martin]

@robert-ros Please, review. The content was updated.