[Gap]: Document CLI support for upstream registry API servers

[danbarr]

As of stacklok/toolhive#2469, the CLI supports upstream registry API servers based on the official spec. This needs to be documented.

[danbarr]

@rdimitrov tagging you on this since you merged the original feature.

[lujunsan]

PR adding these docs is up: #518

[rdimitrov]

Closing as completed via #518