initial COmmit: Add KB Antora Importer plugin files

kb-antora-importer/includes/Frontend/BreadcrumbBuilder.php

@@ -0,0 +1,30 @@
+<?php
+declare(strict_types=1);
+
+namespace KbAntoraImporter\Frontend;
+
+use KbAntoraImporter\Plugin;
+
+final class BreadcrumbBuilder
+{
+    public function build(array $parts): string
+    {
+        $base = trim((string) Plugin::settings()['docs_base_slug'], '/');
+        $items = [
+            sprintf('<a href="%s">%s</a>', esc_url(home_url('/' . $base . '/')), esc_html__('Docs', 'kb-antora-importer')),
+        ];
+        $path = $base;
+
+        foreach ($parts as $label => $slug) {
+            if ('' === (string) $slug) {
+                $items[] = esc_html((string) $label);
+                continue;
+            }
+
+            $path .= '/' . trim((string) $slug, '/');
+            $items[] = sprintf('<a href="%s">%s</a>', esc_url(home_url('/' . $path . '/')), esc_html((string) $label));
+        }
+
+        return '<nav class="kb-breadcrumbs" aria-label="Breadcrumb">' . implode('<span>/</span>', $items) . '</nav>';
+    }
+}