docs: add multilingual sitemap and crawler rules

sitemap.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Author: fruit.com.cn -->
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url><loc>https://ymb.example.com/?lang=id</loc><changefreq>daily</changefreq><priority>1.0</priority></url>
+  <url><loc>https://ymb.example.com/?lang=en</loc><changefreq>daily</changefreq><priority>0.9</priority></url>
+  <url><loc>https://ymb.example.com/?lang=zh</loc><changefreq>daily</changefreq><priority>0.9</priority></url>
+</urlset>