docs(firebase_login): add firebase login example app #5818

Job	Run time
build (stable, examples/counter)	59s
build (stable, examples/marvel)	49s
build (stable, examples/pub)	51s
build (stable, examples/random_number)	47s
build (stable, examples/stackoverflow)	48s
build (stable, examples/todos)	1m 2s
build (stable, packages/riverpod)	1m 44s
build (stable, packages/riverpod/example)	52s
build (stable, packages/flutter_riverpod)	1m 6s
build (stable, packages/flutter_riverpod/example)	58s
build (stable, packages/hooks_riverpod)	58s
build (stable, packages/hooks_riverpod/example)	57s
build (stable, packages/riverpod_annotation)	50s
build (stable, packages/riverpod_generator)	1m 21s
build (stable, packages/riverpod_generator/integration/build_yaml)	1m 16s
build (master, examples/counter)	1m 35s
build (master, examples/marvel)	1m 31s
build (master, examples/pub)	1m 22s
build (master, examples/random_number)	1m 25s
build (master, examples/stackoverflow)	1m 23s
build (master, examples/todos)	1m 43s
build (master, packages/riverpod)	2m 19s
build (master, packages/riverpod/example)	1m 16s
build (master, packages/flutter_riverpod)	1m 48s
build (master, packages/flutter_riverpod/example)	1m 28s
build (master, packages/hooks_riverpod)	1m 34s
build (master, packages/hooks_riverpod/example)	1m 35s
build (master, packages/riverpod_annotation)	1m 11s
build (master, packages/riverpod_generator)	2m 38s
build (master, packages/riverpod_generator/integration/build_yaml)	1m 56s
	40m 2s

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