DOC: Add missing wang2004image and wang2003multiscale BibTeX entries

[Copilot]

itkStructuralSimilarityImageFilter.h cites wang2004image and wang2003multiscale but neither entry existed in Documentation/Doxygen/doxygen.bib, causing nightly doc-build warnings.

Adds both entries in alphabetical order between wallace1996 and warfield2002:

• wang2003multiscale — Wang, Simoncelli, Bovik. "Multiscale structural similarity for image quality assessment." Asilomar Conference on Signals, Systems and Computers, 2003.
• wang2004image — Wang, Bovik, Sheikh, Simoncelli. "Image quality assessment: from error visibility to structural similarity." IEEE Trans. Image Processing 13(4), 2004.

Introduced by #6094.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)

Closes #6179.

[greptile-apps]

Greptile Summary

This PR adds two missing BibTeX entries — wang2003multiscale (MS-SSIM, Asilomar 2003) and wang2004image (SSIM, IEEE TIP 2004) — to Documentation/Doxygen/doxygen.bib, resolving nightly doc-build warnings caused by unresolved \cite references in itkStructuralSimilarityImageFilter.h. The entries are correctly keyed, properly alphabetized between wallace1996 and warfield2002, and the author lists, DOIs, page ranges, and field formatting are consistent with the existing file style.

Confidence Score: 5/5

Safe to merge — purely additive documentation fix with no code changes.

Only one file is touched (doxygen.bib), the change is additive, the new entries are correctly formatted and alphabetically ordered, and the citation keys exactly match the \cite references already present in the codebase.

No files require special attention.

Important Files Changed

Filename	Overview
Documentation/Doxygen/doxygen.bib	Adds two BibTeX entries (wang2003multiscale, wang2004image) in correct alphabetical position between wallace1996 and warfield2002; keys, authors, DOIs, and field formatting match the existing file style.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["itkStructuralSimilarityImageFilter.h\n\\cite wang2004image\n\\cite wang2003multiscale"] -->|resolves via| B["doxygen.bib"]
    B --> C["wang2003multiscale\n@inproceedings — Asilomar 2003\nDOI 10.1109/ACSSC.2003.1292216"]
    B --> D["wang2004image\n@article — IEEE TIP 2004\nDOI 10.1109/TIP.2003.819861"]
    C --> E["Doxygen build — no warnings"]
    D --> E

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