Docs: Add note about Favicon Caching in Vercel #28249

Job	Run time
Determine changes	8s
validate-docs-links	12s
build-native / build	8s
build-next / build	9s
test unit	0s
lint	0s
types and precompiled	0s
rust check	0s
test cargo unit	0s
rustdoc check	0s
test turbopack dev	0s
test turbopack development integration	0s
test next-swc wasi	0s
test next-swc wasm	0s
test turbopack production	0s
test turbopack production integration	0s
test dev	0s
Test new tests for flakes (dev)	0s
Test new tests for flakes (prod)	0s
test firefox and safari	0s
test integration	0s
test ppr dev	0s
test ppr integration	0s
test ppr prod	0s
test prod	0s
report test results to datadog	0s
thank you, next	1s
	38s

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