Updates to features

Add canonical flag, which changes name interpretation
Change feature-set link to feature-sets
Add canonical and kuma-path to feature-sets
Note on links, change browser version ordering

Added notes to links to say which can be changed, which
can not, and what the ordering represents.  Instead
of prev/next links on browser-version objects, the
order in the browser resource determines the version
ordering.
Update link docs, browser-version

* Added notes for each link to indicate ordering and if the user can
change the link or order.
* Dropped previous and next links from browser-version
* Added release-day and retirement-day attributes to browser-version
* Changed current=true to status=current, with other values for
  status ("retired", "beta", "future")
* Added section on sources of version information