DOC: Add note on naming convention inconsistencies (backport #5918)

[mergify]

Pull Request: Clarify Naming Convention Deviations in ROS 2 C++ Style Guide

What was fixed?

• Added a clear notice in the C++ "Variable Naming" section of the documentation.
• The notice explains that ROS 2 projects use a mix of snake_case, PascalCase, and UPPER_CASE for constants and variables, which deviates from the Google C++ Style Guide (which recommends kPascalCase for constants).
• Rationale and guidance were provided for contributors:
  • This deviation is due to historical reasons and consistency with existing ROS codebases.
  • New projects should follow the conventions used in related ROS 2 packages.
  • When in doubt, contributors should prefer consistency with surrounding code over strict adherence to Google style.

Why was this needed?

• There was confusion among contributors about the naming conventions, since the documentation referenced the Google C++ Style Guide but did not explain the deviations.
• This update makes the documentation more transparent and helps maintain consistency across the codebase.

Where was it fixed?

• The change was made in
  source/The-ROS2-Project/Contributing/Code-Style-Language-Versions.rst
  under the C++ > Variable Naming section.

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Example of the new notice:

Note on naming conventions:
ROS 2 deviates from the Google C++ Style Guide in several naming areas:

• The Google style guide recommends kPascalCase for constants (e.g., kDaysInAWeek)
• ROS 2 projects currently use a mix of snake_case, PascalCase, and UPPER_CASE naming conventions
• This deviation is for historical reasons and consistency with existing ROS codebases
• For new projects, developers should follow the existing conventions in related ROS 2 packages
• When in doubt, prefer consistency with surrounding code over strict adherence to Google style

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This change improves clarity for all contributors and reviewers working on ROS 2 C++ code.

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This is an automatic backport of pull request #5918 done by Mergify.

[github-actions]

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