Add proper xkey tags to docs 404 pages

Add proper xkey tags to docs 404 pages

Since we purge docs pages based on the version they are for, we need to
tag the 404 pages with version as well, when available. Without that,
any page that had been requested returning a 404 (such as somebody or
some tool polling for release notes on a version that hasn't been
released yet) would not get purged when new docs are loaded, which
results in the 404 staying around even after the actual docs are
updated.