Expand upon ML explainability

[stevespringett]

As discussed in the ML working groups, we have decided to remove support for explainability due to it being an emerging field of study. At a high level, explainability can be achieved through CDX annotations. Low level explainability is much more complex and will require additional research. This ticket is to track this research and if there is a need to expand upon low-level explainability in a future version of CDX, then we will do that.

This was our work-in-progress snippet that was removed from the schema.

"explainability": {
              "description": "Method of providing some transparency for the model and the reasoning behind generated output",
              "type": "object",
              "additionalProperties": false,
              "properties": {
                "method": {
                  "description": "TODO - Christian to research",
                  "type": "array"
                },
                "method type": "model explainability vs input explainability"
                "todo" : "add description, add evidence (graphics, attachments), external reference for explainability"
              }
            },