[DOCS] Mention XML sitemap configuration (#735)

Documentation/Configuration/XmlSitemap.rst

@@ -0,0 +1,24 @@
+.. _xmlsitemap:
+
+===================
+XML Sitemap
+===================
+
+Since `ext:headless` 4.0 there's no custom code regarding the XML sitemaps
+apart from the templates used to render the XML sitemap's source code.
+
+Troubleshooting
+===============
+
+In case the URLs listed in the "sitemap index" file (usually `/sitemap.xml`)
+aren't "frontend URLs" (but their API variant), consider setting one of the two
+following configurations:
+
+* either `frontendApiProxy` being defined in the site's `config.yaml`
+* or this in the site's `settings.yaml`:
+
+.. code-block:: yaml
+
+  headless:
+    sitemap:
+      key: frontendBase

Documentation/Index.rst

@@ -72,6 +72,7 @@ or `report a problem <https://github.com/TYPO3-Initiatives/headless/issues/new>`
    Installation/Index
    Configuration/Index
    Configuration/Preview
+   Configuration/XmlSitemap
    Developer/Index
    Developer/Images
    FAQ/Index