Use shields.io for DOI badge to avoid Zenodo badge timeouts

[MedantSharan]

Zenodo’s DOI badge endpoint is unreliably slow, causing GitHub to render fallback text (“DOI”).
This PR switches the badge image to shields.io (already used elsewhere in the README) while keeping the DOI link unchanged.

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[github-actions]

⚠️ Branch Update Required

Your branch is 1 commits behind main. Please update your branch to ensure CI checks run with the latest code:

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Note: Enable "Allow edits by maintainers" to allow automatic updates.