Added gz_sim_spray_painting_plugin to projects list

[topguns837]

🎉 New feature

Summary

Adds gz_sim_spray_painting_plugin to the Gazebo Plugins section of projects_using_gazebo.md.

This plugin simulates spray painting through ray-casting and particle emitter visual effects, helping users visualize paint coverage for autonomous spray painting applications.

Plugin repo: https://github.com/topguns837/gz_sim_spray_painting_plugin

Backport Policy

• This should not be backported

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature — N/A (docs-only change)
• Added tests — N/A
• Added example and/or tutorial — N/A
• Updated documentation (as needed) — this PR is the documentation update
• Updated migration guide (as needed) — N/A
• Consider updating Python bindings — N/A
• codecheck passed — N/A (no code changes)
• All tests passed — N/A
• Updated Bazel files — N/A
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? Yes — see below.

Generated-by: Claude Sonnet 4.6 (entry description drafted with AI assistance)