DOC: add extra development section

[hagenw]

Move changelog and contributing section to extra part in docs as in other packages:

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[sourcery-ai]

Hey @hagenw - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟡 General issues: 1 issue found
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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