Docs: Create documentation for the Greenshot plugin

[rj9884]

Related Issue

Closes #448

Proposed Changes

• Created comprehensive documentation file for the Greenshot plugin at docs/plugins/greenshot.md.
• Added registration entry for the greenshot plugin to the "Automation, Productivity, and Desktop Utilities" table in docs/plugins/README.md.
• Documented configuration format, INI section mapping (backslash-separated compound keys), common settings, and verified usage examples.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 📖 Documentation
• 🧪 Testing
• 🛠️ Refactoring
• 🚀 DevOps/CI

Screenshots / Logs (if applicable)

Verified configuration examples against the plugin parser engine (plugins/greenshot/src/plugin.py) using a python validation script:

Example 1 result: {'requestId': 'test-1', 'changed': True}
Example 2 result: {'requestId': 'test-2', 'changed': True}
Example 3 result: {'requestId': 'test-3', 'changed': True}
All examples successfully validated against the plugin engine!

Testing & Verification

• I have run dotnet test and all 60+ cross-platform tests passed. (Verified plugin unit tests locally)
• I have verified the changes on a Windows environment (if applicable).
• I have added new unit tests to cover my changes.

GSSOC 2026 Checklist

• I have read the Contribution Guidelines.
• My code is formatted correctly (I have run dotnet format, uv tool run ruff format plugins/, and bun x prettier --write plugins/).
• I have linked the PR to an approved issue.
• I understand that a maintainer must apply the gssoc:approved label for this PR to count for points.

[DotDev262]

Clean Greenshot docs — standard template, POSIX newlines, scoped. Approved.