docs: add better-auth auth provider #49850

Job	Run time
Determine changes	8s
validate-docs-links	14s
optimize-ci / Graphite CI Optimizer	3s
build-native / build	7s
build-next / build	10s
test unit	0s
test devlow package	0s
lint	0s
types and precompiled	0s
ast-grep lint	0s
Run devlow benchmarks	0s
rust check	0s
rustdoc check	0s
test cargo benches	0s
test cargo unit	0s
test dev	0s
Test new tests for flakes (dev)	0s
Test new tests for flakes (prod)	0s
test next-swc wasi	0s
test next-swc wasm	0s
test prod	0s
test turbopack dev	0s
test turbopack development integration	0s
test turbopack production	0s
test turbopack production integration	0s
test firefox and safari	0s
test integration	0s
test ppr dev	0s
test ppr integration	0s
test ppr prod	0s
Test new tests when deployed	0s
report test results to datadog	0s
thank you, next	1s
	43s

Provide feedback