Merge bitcoin/bitcoin#31007: doc: add testnet4 section header for con… #1488

Job	Run time
macOS 14 native, arm64, no depends, sqlite only, gui	0s
Win64 native, VS 2022	0s
ASan + LSan + UBSan + integer, no depends, USDT	0s
test each commit	0s
	0s

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