docs(demo): add "what it is / when to use" captions to demo section groups #1560

Job	Run time
Verify all generated artifacts	23s
Lint CSS	11s
Build bundle	17s
Check migration docs for breaking changes	6s
Lint commit messages	15s
Dependency vulnerability audit	9s
Regression tests	5m 48s
Configurator tests	2m 25s
	9m 34s

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