Update references to DID Specification Registries throughout

[wip-abramson]

We need to do an editorial pass to update DID Specification Registries to the appropriate extensions list. Most commonly I think this would be the DID Resolution Extensions.

I think it is better to individually reference each of the extensions, e.g. https://www.w3.org/TR/did-extensions-resolution/.

Rather than pointing to the overall extensions document - https://w3c.github.io/did-extensions.

One thing I am unsure about is how the reference [[?DID-SPEC-REGISTRIES]] is defined/generated. Couldn't find it in the respecConfig, so is it being pulled from some other document? cc @msporny