Adding book cover page image into the documentation page.

[pandyasio]

What is this PR about?
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Checklist

• Explain important implementation details 👆
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Major / Breaking Changes

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New features

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Bugfixes

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Documentation

• Fixed book cover pages in resources section.

Maintenance

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📚 Documentation preview 📚: https://pymc--6683.org.readthedocs.build/en/6683/

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[twiecki]

Thanks @PlayingData -- this is great!