Add/designate sample contexts as being stable for long-term reuse by testsuites/example apps

[ansell]

@davidlehn informs me that the files in https://github.com/json-ld/json-ld.org/tree/master/contexts were not formally expected to be used by testsuites or example applications in the long term. I don't mind if they disappear (my testsuites will break temporarily but no other harm done), but it would be great to either formally recognise the long term use of those static contexts (5+ years in the case of person.jsonld) in their current locations, or create a new long-term test location. My preference is for keeping the existing paths (less work for everyone).