Docs: add config.blocks.initialBlocks documentation #20012

Job	Run time
Core Basic (20.x)	7m 59s
Core Blocks (20.x)	8m 17s
Core Basic (22.x)	7m 4s
Core Blocks (22.x)	8m 6s
Core Control Panels (20.x)	4m 17s
Core Basic - Plone 5 (22.x)	6m 24s
Core Control Panels (22.x)	4m 59s
Coresandbox (22.x)	5m 28s
Core Content (20.x)	3m 3s
Core Blocks - Listing (20.x)	6m 7s
Core Content (22.x)	2m 58s
Core Volto Slate (20.x)	7m 11s
Core Blocks - Listing (22.x)	4m 29s
Deployment Mode (22.x)	6m 8s
Core Volto Slate (22.x)	7m 10s
Project Generator (20.x)	6m 16s
Multilingual (22.x)	2m 50s
Project Generator (22.x)	6m 12s
Multilingual in Deployment Mode (22.x)	3m 38s
Working Copy (22.x)	3m 23s
	1h 51m 59s

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