docs: add note for environment variables on Vercel deployment #16848

Job	Run time
validate-docs-links	10s
Determine changes	7s
build-native / build	20s
build-next / build	28s
test unit	0s
test cargo integration	0s
types and precompiled	0s
test cargo unit	0s
rust check	0s
test next-swc wasm	0s
test turbopack dev	0s
test turbopack integration	0s
test dev	0s
test firefox and safari	0s
test integration	0s
test ppr dev	0s
test ppr integration	0s
test ppr prod	0s
test prod	0s
lint / build	1m 11s
test cargo benchmarks / build	32s
report test results to datadog	0s
thank you, next	1s
	2m 49s

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