Update built-in registry references to reflect move to separate repo

[danbarr]

The built-in registry is now maintained in the toolhive-registry repo. There are some references to registry.json in the docs that should be updated.

docs-website/docs/toolhive/concepts/registry-criteria.md

Lines 22 to 24 in 8764141

	Registry entries are defined in the
	[`pkg/registry/data/registry.json`](https://github.com/stacklok/toolhive/blob/main/pkg/registry/data/registry.json)
	file in the ToolHive repository.

docs-website/docs/toolhive/guides-cli/registry.md

Lines 140 to 142 in 8764141

	Refer to the
	[built-in registry file](https://github.com/stacklok/toolhive/blob/main/pkg/registry/data/registry.json)
	for examples of MCP server entries.

(note this one is probably OK to keep because it still does reflect the final assembled registry JSON)