docs: add a version switch. #3810

Job	Run time
smoke (amaranth-lang/amaranth-soc)	1m 5s
smoke (amaranth-lang/amaranth-stdio)	1m 4s
test (3.13)	3m 30s
test (3.11)	2m 50s
test (3.12)	2m 58s
test (3.14)	2m 50s
document	1m 2s
test (3.10)	2m 34s
check-links	1m 0s
smoke (amaranth-lang/amaranth-boards)	39s
test (3.9)	2m 42s
test (pypy-3.11)	3m 56s
test (pypy-3.10)	4m 57s
test (pypy-3.9)	4m 45s
test (3.15-dev)	1m 56s
publish-docs	5s
publish-docs-dev	-1s
required	4s
publish-package	16s
publish-schemas	9s
publish-release	0s
	38m 21s

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