cli, docs: Add a docs command for doc generation

This took a lot longer than it looks to get right. It's not perfect, but
it now reliably generates documentation which we can put into gohugo.

test/test-docs-generate.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+# check that our documentation still generates, even if we don't use it here
+
+# shellcheck disable=SC1091
+. test/util.sh
+
+echo running "$0"
+
+ROOT=$(dirname "${BASH_SOURCE}")/..
+cd "${ROOT}"
+
+failures=''
+
+run-test ./mgmt docs generate --output /tmp/docs.json &> /dev/null || fail_test "could not generate: $file"
+
+if [[ -n "$failures" ]]; then
+	echo 'FAIL'
+	echo "The following tests have failed:"
+	echo -e "$failures"
+	echo
+	exit 1
+fi
+echo 'PASS'