docs: update CONTRIBUTING with v2 branching strategy #3919

Job	Run time
checks / pre-commit	34s
checks / readme-snippets	14s
checks / test (3.11, lowest-direct, windows-latest)	2m 26s
checks / test (3.10, lowest-direct, windows-latest)	2m 15s
checks / test (3.10, highest, ubuntu-latest)	47s
checks / test (3.11, highest, ubuntu-latest)	48s
checks / test (3.10, lowest-direct, ubuntu-latest)	45s
checks / test (3.12, highest, windows-latest)	3m 29s
checks / test (3.13, lowest-direct, windows-latest)	2m 40s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.12, lowest-direct, windows-latest)	3m 25s
checks / test (3.13, highest, ubuntu-latest)	51s
checks / test (3.12, highest, ubuntu-latest)	50s
checks / test (3.11, lowest-direct, ubuntu-latest)	46s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.13, highest, windows-latest)	2m 29s
checks / test (3.10, highest, windows-latest)	2m 17s
checks / test (3.11, highest, windows-latest)	2m 46s
	29m 10s

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