Add sitemap.xml to make Relay Search pages indexed by Google et al.
Right now, Google doesn't index Relay Search details pages.
One way to provide this file is to fetch the full https://onionoo.torproject.org/summary and generate the details page URLs from that. Caching the Onionoo summary file and using the If-Modified-Modified header would be very much appreciated here.
The specification for sitemap.xml can be found at:
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