Serve v2 docs at the site root, with permanent per-major paths #3666

Job	Run time
checks / docs	1m 7s
checks / pre-commit	55s
checks / readme-snippets	15s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 24s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.11, lowest-direct, windows-latest)	1m 55s
checks / test (3.10, locked, windows-latest)	1m 59s
checks / test (3.14, lowest-direct, windows-latest)	2m 1s
checks / test (3.13, locked, windows-latest)	2m 12s
checks / test (3.12, lowest-direct, windows-latest)	1m 57s
checks / test (3.13, locked, ubuntu-latest)	1m 2s
checks / test (3.11, locked, ubuntu-latest)	1m 28s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.10, lowest-direct, windows-latest)	1m 45s
checks / test (3.10, locked, ubuntu-latest)	1m 13s
checks / test (3.14, locked, ubuntu-latest)	1m 19s
checks / test (3.12, locked, windows-latest)	1m 57s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 29s
checks / test (3.12, locked, ubuntu-latest)	1m 33s
checks / test (3.13, lowest-direct, windows-latest)	2m 13s
checks / test (3.11, locked, windows-latest)	1m 52s
checks / test (3.14, locked, windows-latest)	1m 28s
all-green	5s
	35m 5s

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