docs: adding how to add language packs in autoinstall.yaml #2825

Job	Run time
flutter-format	2m 12s
bootstrap (ubuntu_bootstrap_test.dart)	10m 40s
flutter-mocks	5m 15s
bootstrap (screenshot_test.dart)	10m 7s
flutter-analyze	2m 39s
flutter-tests	8m 20s
Go: Code sanity	1m 55s
Go: Tests	2m 11s
init (ubuntu_init_test.dart)	2m 33s
init-e2e	5m 23s
init (screenshot_test.dart)	3m 40s
l10n	1m 27s
build-and-upload	0s
	56m 22s

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