Add Type annotations developer guideline

[Avasam]

Summary of changes

Add Type annotations developer guideline CC @abravalheri feel free to wordsmith. I figured these things are probably better documented with sources than left in PR discussions.

Pull Request Checklist

• [x] Changes have tests (N/A)
• [x] News fragment added in newsfragments/. (N/A) (See documentation for details)

[Avasam]

@abravalheri Updated and added a few more details !

[abravalheri]

Thank you very much.