Finalize JSON-LD context files for REC

[msporny]

This is an issue to make sure we remember to do the following when DID v1.1 is published as a REC:

https://www.w3.org/ns/did/v1.1 should serve the final v1.1 JSON-LD context as a static file directly from the W3 servers (it should be locked in and not changed -- and not a redirect). This is to ensure that someone that makes a mistake on the Github repo doesn't then cause a static, REC-locked file to accidentally change (just like we don't update historical TR documents in place unless there are serious extenuating circumstances).