Align `abbrev` with CIMD as is commonly used

[ThisIsMissEm]

Currently the abbrev in the draft's metadata is "Client ID Document", we should probably align this to be CIMD, as that's what we've started using everywhere (e.g., https://cimd.dev ). The existing abbrev also missing the "Metadata" which is kind of important.

I'm not sure we can change this now, but if we can, it'd be good to.

[aaronpk]

I don't think it's a problem to change it