add explanation of rcl_logging_implementation.

[fujitatomoya]

Description

Part of ros2/rcl#1178

Did you use Generative AI?

Yes, Claude Sonnet 4.0

Additional Information

[Copilot]

Pull request overview

This PR adds comprehensive documentation for the rcl_logging_implementation abstraction layer in ROS 2's logging subsystem. The documentation explains how ROS 2 supports both dynamic loading and static linking of logging backends, similar to the rmw_implementation pattern used for middleware selection.

Changes:

• Added documentation for the RCL_LOGGING_IMPLEMENTATION environment variable in the environment variables section
• Updated the logging subsystem design description to clarify the two linking modes (dynamic and static)
• Added a new comprehensive section explaining rcl_logging_implementation, including runtime dynamic loading vs static linking, environment variable configuration, implementation details, and use cases

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/21190243959/artifacts/5196996059.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-6166/index.html in your favorite browser

[peci1]

The proposed docs change looks good! However, I think it's also needed to change the diagram showing the library relations and add rcl_logging_implementation to it (and maybe also the two default logging libraries).

[fujitatomoya]

@peci1 ah yeah, right. i will replace that into the correct stack diagram. thanks 👍

[fujitatomoya]

@Yadunund @Barry-Xu-2018 workflow is green, can you review this?

[Barry-Xu-2018]

LGTM