Convert README.md links to absolute URLs with reference-style syntax

[Copilot]

The README.md is packaged in the NuGet distribution and displayed on nuget.org, where relative links to repository files fail.

Changes

• Converted relative links (CONTRIBUTING.md, CODE_OF_CONDUCT.md, ARCHITECTURE.md) to absolute GitHub URLs
• Migrated all links to reference-style syntax with definitions at file bottom for centralized maintenance

Example

Before:

Please see our [Contributing Guide](CONTRIBUTING.md) for details.

After:

Please see our [Contributing Guide][contributing] for details.

[contributing]: https://github.com/demaconsulting/SpdxTool/blob/main/CONTRIBUTING.md

All links now resolve correctly on nuget.org while maintaining easy updateability through the centralized reference definitions.

Original prompt

please modify the README.md file to make all the links absolute so they will work when the readme file is put in the nuget package and viewed on nuget.org. the links should use Markdown link references for easy maintenance.

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