Adding book cover page image into the documentation page.

[heypandya]

What is this PR about?
...

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• ...

New features

• ...

Bugfixes

• ...

Documentation

• Fixed book cover pages in resources section.

Maintenance

• ...

---

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

[welcome]

💖 Thanks for opening this pull request! 💖 The PyMC community really appreciates your time and effort to contribute to the project. Please make sure you have read our Contributing Guidelines and filled in our pull request template to the best of your ability.

[welcome]

Congrats on merging your first pull request! 🎉 We here at PyMC are proud of you! 💖 Thank you so much for your contribution 🎁

[twiecki]

Thanks @PlayingData -- this is great!