Add sitemap.xml for search engines

- Create sitemap.xml with main sections of the archive site
- Add Sitemap reference to robots.txt
- All URLs marked as never-changing (archive content)
- Helps search engines index the legacy frameset site properly

html/robots.txt

@@ -1 +1,5 @@
 User-agent: *
+Allow: /
+
+# Sitemap для поисковиков
+Sitemap: https://1998.cadpoint.ru/sitemap.xml