Include the absolute URL to the sitemap in the robots.txt file (#633)
In the robots.txt file, the value of the sitemap field is a relative
URL.
This should be changed to an absolute URL.
See protocol: https://sitemaps.org/protocol.html#submit_robots
For example, in the Google search console, this error message appears
when it's a relative url:
> "Invalid sitemap URL" incomprehensible syntax
I think it would be interesting to have a default sitemap field in the
right format using `root_url`.
François Bruynbroeck