Update install.rst to specify how to install board definitions #3175

Job	Run time
document	44s
check-links	3m 43s
test (3.8)	1m 54s
smoke (amaranth-lang/amaranth-boards)	47s
test (3.9)	1m 43s
smoke (amaranth-lang/amaranth-stdio)	33s
test (3.10)	2m 14s
smoke (amaranth-lang/amaranth-soc)	40s
test (3.11)	2m 14s
test (3.12)	2m 53s
test (pypy-3.8)	5m 23s
test (pypy-3.9)	4m 58s
test (pypy-3.10)	4m 59s
test (3.13-dev)	2m 27s
publish-docs	6s
publish-docs-dev	0s
required	1s
publish-package	30s
publish-schemas	4s
publish-release	0s
	35m 53s

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