Would be nice if Securing Mechanisms mapped media type to spec section

[jyasskin]

When tracing the control flow from https://w3c.github.io/vc-data-model/#verification to a particular securing algorithm, I have an input media type and the table in https://w3c.github.io/vc-specs-dir/#securing-mechanisms, and it can be hard to find the function that verifyProof gets set to. I think it'd be easier if the table mentioned the media type(s) for each spec, and the section of the spec that defines the function needed for each media type.