Refer to central contributing guide and add code of conduct

[tovrstra]

This is my suggestion to refer to the contributing guide in the .github repository proposed in theochem/.github#6

For the bigger picture, see #313

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Here's the code health analysis summary for commits d4bb73f..ba2ca35. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Shell	✅ Success		View Check ↗
Python	✅ Success		View Check ↗

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[tovrstra]

Tests fail due to the recent release of NumPy 2. We'll need to fix this first to get all tests on the rails again.

[PaulWAyers]

Thanks Toon.

[tovrstra]

Thanks for reviewing...