add(book): Adds a section on Regtest usage to the Zebra book #1323

Job	Run time
Generate crates matrix	12s
Check crates matrix	6s
Build zebrad crate	35m 27s
Build zebra-utils crate	18m 47s
Build zebra-test crate	1m 31s
Build zebra-state crate	22m 8s
Build zebra-script crate	4m 49s
Build zebra-scan crate	20m 57s
Build zebra-rpc crate	24m 46s
Build zebra-node-services crate	3m 15s
Build zebra-network crate	6m 52s
Build zebra-grpc crate	14m 40s
Build zebra-consensus crate	19m 9s
Build zebra-chain crate	5m 1s
Build tower-fallback crate	59s
Build tower-batch-control crate	1m 21s
Open or update issues for building crates individually failures	0s
	3h 0m 0s

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