[buck2] add platform-independent buck2 installation instructions (#402) #2362

Job	Run time
test-matrix (macos, arm64, sonoma, 14, 8c27ba1624955003040780ea24a0ec25a3b5e309, false)	50s
test-matrix (macos, arm64, sonoma, 14, 8c27ba1624955003040780ea24a0ec25a3b5e309, true)	41s
ci-runners-test-matrix (false)	4m 5s
ci-runners-test-matrix (true)	1m 5s
buck2-test	18s
	6m 59s

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