feat: add sitemap for help center (#9280)

* feat: start sitemap

* feat: add base url and last mod to sitemap

* fix: typo

* test: sitemap generation

* test: add draft articles

* fix: escape dots in regex matching

* feat: perpend protocol to the url

* feat: use ChatwootApp.help_center_root

* feat: don't parse the URL

* fix: function declaration

app/views/public/api/v1/portals/sitemap.xml.erb

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<sitemapindex>
+  <% @portal.articles.where(status: :published).each do |article| %>
+    <sitemap>
+      <loc><%= @help_center_url %><%= generate_article_link(@portal.slug, article.slug, false, false) %></loc>
+      <lastmod><%= article.updated_at.strftime("%Y-%m-%d") %></lastmod>
+    </sitemap>
+  <% end %>
+</sitemapindex>