docs: update CONTRIBUTING with v2 branching strategy (#1804) #534

Job	Run time
checks / pre-commit	38s
checks / readme-snippets	14s
checks / test (3.11, lowest-direct, windows-latest)	2m 20s
checks / test (3.10, lowest-direct, windows-latest)	2m 16s
checks / test (3.11, lowest-direct, ubuntu-latest)	47s
checks / test (3.10, highest, ubuntu-latest)	45s
checks / test (3.10, lowest-direct, ubuntu-latest)	45s
checks / test (3.11, highest, windows-latest)	2m 53s
checks / test (3.12, highest, windows-latest)	3m 24s
checks / test (3.11, highest, ubuntu-latest)	49s
checks / test (3.12, highest, ubuntu-latest)	50s
checks / test (3.13, lowest-direct, windows-latest)	2m 34s
checks / test (3.13, lowest-direct, ubuntu-latest)	43s
checks / test (3.13, highest, windows-latest)	2m 51s
checks / test (3.13, highest, ubuntu-latest)	45s
checks / test (3.10, highest, windows-latest)	2m 14s
checks / test (3.12, lowest-direct, ubuntu-latest)	51s
checks / test (3.12, lowest-direct, windows-latest)	3m 37s
	29m 16s

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