Showcase - Add a frameless example for the `AppSideNav` #15125

Job	Run time
Get files changed and conditionally skip CI	14s
Tests	9m 3s
ember-lts-3.28	7m 14s
ember-lts-4.12	4m 43s
ember-beta	4m 38s
embroider-safe	3m 59s
embroider-optimized	1m 41s
	31m 32s

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