DOC: add and clarify guidelines for referencing external code

[devesh-047]

This PR expands the existing contributor guidance on acknowledging external code by
adding a dedicated section that explains when attribution is required, where references
should be placed, recommended formats, and licensing considerations.

A concrete example (PR #2748) is included for clarity. The documentation builds
successfully and renders correctly.

Closes #3213.

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[devesh-047]

I’ve opened a PR adding explicit guidelines for referencing external code, aligned with
the example in PR #2748.

[MatthewMiddlehurst]

Probably a bit much for the contributor guide pages. It appears to confuse paper references with the attribution from the PR I made? Not sure if generated by an LLM

Please dont ignore the template and contributor guide.