`namespace` entries should point to a url of a JSON list of decision points in that namespace

[ahouseholder]

@tschmidtb51 suggested this in a comment on #703:

I'm currently implementing SSVC in CSAF. My suggestion is to have a registry of namespaces. The registry should contain the namespace and a url of the JSON list containing the available decision points. Such registry would really help with our test 6.1.45 and 6.2.33

Originally posted by @tschmidtb51 in #703

This issue captures an extension to #749.

[ahouseholder]

Copying example from @tschmidtb51's comment in #824:

https://github.com/CERTCC/SSVC/pull/824/files#r2216583416

{
   "$schema": "https://raw.githubusercontent.com/certcc/ssvc/main/data/schema/registered_namespaces.json",
   "last_updated": "2025-07-19T12:00:00+02:00",
   "namespaces": [
        {
                "namespace": "cvss",
                "maintained_by": "FIRST",
                "decision_point_url": "https://github.com/CERTCC/SSVC/tree/main/data/json/decision_points/cvss",
                "description": "The CVSS namespace contains all decision points of any CVSS standard version [...]"
        },
        {
                "namespace": "ssvc",
                "maintained_by": "CERT/CC",
                "decision_point_url": "https://github.com/CERTCC/SSVC/tree/main/data/json/decision_points/ssvc",
                "description": "The SSVC namespace contains all decision points of any SSVC standard version [...]"
        }
    ]
}