docs: document trait declaration deprecation 🤖(#520)

[frobel0520]

Summary

• Document that trait declarations should use instances in the migration notes.
• Add examples for the deprecated and preferred forms.

Closes #520

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.19%. Comparing base (5b398be) to head (8fddd8d).

Components	Coverage Δ
traitlets	85.60% <ø> (ø)
tests	99.07% <ø> (ø)

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