Move table of contents styling into concepts #347

Job	Run time
Percy Tests	3m 47s
Floating Dependencies	2m 2s
Tests	3m 56s
ember-lts-3.28	3m 29s
ember-canary - Allowed to fail	3m 54s
ember-release	3m 21s
ember-release-no-deprecations - Allowed to fail	3m 30s
ember-beta	3m 23s
ember-classic	4m 0s
ember-default-with-jquery	3m 20s
embroider-safe	4m 10s
embroider-optimized	3m 22s
no-deprecations	4m 17s
	46m 31s

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