add authentication docs page #18846

Job	Run time
Determine changes	10s
validate-docs-links	14s
build-native / build	38s
build-next / build	30s
test unit	0s
rust check	0s
test cargo unit	0s
test turbopack dev	0s
test turbopack integration	0s
test next-swc wasm	0s
test dev	0s
types and precompiled	0s
test integration	0s
test prod	0s
test ppr integration	0s
test firefox and safari	0s
test ppr dev	0s
test ppr prod	0s
lint / build	1m 12s
report test results to datadog	0s
thank you, next	1s
	2m 45s

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