initial COmmit: Add KB Antora Importer plugin files

kb-antora-importer/includes/Import/Checksum.php

@@ -0,0 +1,12 @@
+<?php
+declare(strict_types=1);
+
+namespace KbAntoraImporter\Import;
+
+final class Checksum
+{
+    public static function content(string $content): string
+    {
+        return hash('sha256', $content);
+    }
+}