Set value for sitemap_url_scheme to correctly add URL entries to sitemap.xml.

Currently we have values that follow the default settings value, such as the following.
`<loc>https://6.docs.plone.org6.0/backend/annotations.html</loc>`
This change fixes the issue.

Author: stevepiercy

docs/conf.py

@@ -278,7 +278,9 @@
 html_use_index = True
 
 # Used by sphinx_sitemap to generate a sitemap
-html_baseurl = "https://6.docs.plone.org"
+html_baseurl = "https://6.docs.plone.org/"
+# https://sphinx-sitemap.readthedocs.io/en/latest/advanced-configuration.html#customizing-the-url-scheme
+sitemap_url_scheme = "{link}"
 
 # -- Options for HTML help output -------------------------------------------------