DOC: Add page for v4-migration guide #1377

Job	Run time
py3.11 \| ubuntu \| unit tests	4m 23s
py3.12 \| ubuntu \| unit tests	4m 33s
py${{ matrix.python-version }} \| ${{ matrix.os }} \| integration tests	-1s
mypy	-1s
merge-test-artifacts	0s
	8m 54s

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