Add first_rclrs_node example and update documentation links

[roboticswithjulia]

Merge Request: Add Improvements to Documentation in first_rclrs_node.rs

Description

This merge request moves the previous documentation written in docs/writing-your-first-rclrs-node.md into a custom example documentation ./examples/minimal_pub_sub/src/first_rclrs_node.rs. The changes aim to enhance clarity, readability and integration with Rust ecosystem.

Changes

• Code has been updated with the latest changes in the rclrs crate, corresponding with the current repository.
• Documentation has been added following the https://github.com/ros2-rust/ros2_rust/issues/465#issuecomment-2740400527 indication. Thank you @mxgrey!
• New node has been added in minimal_pub_sub, which allows also to compile the example.

Modified Files

• writing-your-first-rclrs-node.md [Deleted]
• first_rclrs_node.rs
• Readme.md

How to Test

1. Review the changes in the first_rclrs_node.rs file.
2. Ensure that the instructions are clear and easy to follow.

Additional Notes

No functional code changes were made; only documentation improvements were included in this merge request.

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Please review the changes and provide feedback or approval.