add(book): Describes custom Testnets in the Zebra book #1545

Job	Run time
Check deny.toml bans	27s
Check Cargo.lock is up to date	57s
Check deny.toml bans --features default-release-binaries	27s
Check deny.toml bans --all-features	27s
Check deny.toml sources	27s
Check deny.toml sources --features default-release-binaries	28s
Install zebrad from lockfile without cache on ubuntu-latest	8m 25s
Check deny.toml sources --all-features	27s
Test stable on ubuntu-latest	28m 55s
Test beta on ubuntu-latest	28m 9s
Test stable on macos-latest	29m 55s
Check for unused dependencies	8s
Test stable on windows-latest	55m 43s
Test beta on windows-latest	56m 27s
Open or update issues for OS integration failures	0s
	3h 31m 22s

Provide feedback