docs: add note that contentlayer is unmaintained #17380

Job	Run time
Determine changes	7s
validate-docs-links	15s
build-native / build	19s
build-next / build	22s
test unit	0s
types and precompiled	0s
test cargo integration	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 integration	0s
test prod	0s
test firefox and safari	0s
test ppr dev	0s
test ppr integration	0s
test ppr prod	0s
lint / build	1m 9s
test cargo benchmarks / build	31s
report test results to datadog	0s
thank you, next	1s
	2m 44s

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