doc(help): Add section for help extensions #750

Job	Run time
pkryger-taps (macos-latest, 29.4)	3m 18s
ci-tests (ubuntu-latest, 28.1)	2m 2s
pkryger-taps (macos-latest, snapshot)	2m 59s
ci-tests (ubuntu-latest, 28.2)	2m 5s
ci-tests (ubuntu-latest, 29.1)	2m 1s
ci-tests (ubuntu-latest, 29.2)	2m 12s
ci-tests (ubuntu-latest, 29.3)	2m 10s
ci-tests (ubuntu-latest, 29.4)	3m 36s
ci-tests (ubuntu-latest, snapshot)	3m 3s
ci-tests (macos-latest, 28.1)	3m 7s
ci-tests (macos-latest, 28.2)	3m 19s
ci-tests (macos-latest, 29.1)	2m 43s
ci-tests (macos-latest, 29.2)	2m 56s
ci-tests (macos-latest, 29.3)	3m 2s
ci-tests (macos-latest, 29.4)	4m 33s
ci-tests (macos-latest, snapshot)	4m 2s
	47m 8s

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