Enable other DID doc content types

[rhiaro]

A hardcoded file called did.json is not very flexible, and excludes other representations. We should update the spec to use Accept headers on Read. It'd also be better if the requester for Read just fetches the DID as it stands without adding fixed strings or filenames on the end.

[dmitrizagidulin]

Hmmm, yeah, maybe we can design a mechanism that allows for elegant DIDs without filenames, but also allows for filenames with extensions optionally. (Although there aren't really other representations to exclude, in this method (it defines only the JSON/JSON-LD representation).

-1 to using Accept headers. (Many many web servers aren't able to do content negotiation.)