feat: Create sitemap.xml

Creating sitemap index

See: https://developers.google.com/search/docs/advanced/sitemaps/large-sitemaps

It appears Vale at Red Hat is the only project with a sitemap: https://redhat-documentation.github.io/vale-at-red-hat/sitemap.xml.

Author: themr0c

sitemap.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <sitemap>
+    <loc>https://redhat-documentation.github.io/vale-at-red-hat/sitemap.xml</loc>
+  </sitemap>
+</sitemapindex>