Add some docs on the _meta /publisher-provided field

[tadasant]

I like our solution to https://github.com/modelcontextprotocol/registry/issues/356

We could do a little more to document it.

The core shape is:

{
  "name": "my-server",
  "description": "...",
  "_meta": {
    "io.modelcontextprotocol.registry/official": {
        /* Have all official registry extensions grouped */
        "id": "b6c3a9d5-aa7c-4520-bf5c-b484731bb5b0",
        "published_at": "2025-09-04T14:37:54.899304Z",
        "updated_at": "2025-09-04T14:37:54.899304Z",
        "is_latest": true
     },
    "io.modelcontextprotocol.registry/publisher-provided": {
        /* Have the existing custom metadata, currently advertised via publisher here */
        "is_organisation": true,
        ...
        "license": "MIT License"
      }
   }
}

I think I would go a step further and recommend folks do something like this (I believe @rdimitrov hinted at this suggestion to @toby in Discord):

{
  "name": "my-server",
  "description": "...",
  "_meta": {
    "io.modelcontextprotocol.registry/official": {
        /* Have all official registry extensions grouped */
        "id": "b6c3a9d5-aa7c-4520-bf5c-b484731bb5b0",
        "published_at": "2025-09-04T14:37:54.899304Z",
        "updated_at": "2025-09-04T14:37:54.899304Z",
        "is_latest": true
     },
    "io.modelcontextprotocol.registry/publisher-provided": {
        /* Have the existing custom metadata, currently advertised via publisher here */
        "com.github": {
          "serverDisplayName": "My Server"
        }
      }
   }
}

i.e. discourage non-namespaced entries inside publisher-provided.