Consider the following links:
As you can see, the same template data is being associated to more than one template:
1) On enwiki, both "Template:Foo" and "Template:Foo/doc"
2) On ptwiki, both "Template:Foo", "Template:Foo/doc" and "Template:Foo/dados"
Using <includeonly><templatedata>...JSON...</templatedata></includeonly> on "/doc" and "/dados" would make it impossible to preview the table in the same page.
I think a better approach would be to have a specific namespace, say "JSON:" (take this just as an example) such that "JSON:XYZ" would hold the JSON for "Template:XYZ" (and no other template). So it could be transcluded to "/doc" subpages IF the users want that table in the middle of the documentation (which is not always desirable, since it seems a little technical for newbie users, repetitive - because of the template name, its label and its description - and will be too big for templates with a lot of parameters and aliases)