Docs: Add note on 'build targets' on Get Started #3425

Job	Run time
Run unit tests	1m 9s
Test plugin scaffolding (myorg-nobackend-app, create-plugin --pluginName='no-backend' --orgName='...	2m 19s
Test plugin scaffolding (myorg-backend-app, create-plugin --pluginName='backend' --orgName='myorg...	3m 16s
Test plugin scaffolding (myorg-nobackend-panel, create-plugin --pluginName='no-backend' --orgName...	2m 13s
Test plugin scaffolding (myorg-nobackend-datasource, create-plugin --pluginName='no-backend' --or...	2m 21s
Test plugin scaffolding (myorg-backend-datasource, create-plugin --pluginName='backend' --orgName...	3m 8s
Test plugin scaffolding (myorg-nobackendscenes-app, create-plugin --pluginName='no-backend-scenes...	2m 8s
Release packages	49s
	17m 23s

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