Document server.json

[tadasant]

We have a starting point for this, but we should:

• Formally define what is and is not valid in this JSON
• Make sure we have some mechanism for versioning the schema, because it's very likely to evolve over time (especially as non-registry initiatives start to prioritize defining the notion of static server data)

[tadasant]

Note: we should be thoughtful about how to define version as per https://github.com/modelcontextprotocol/registry/pull/134/files#r2145287494

[tadasant]

Also make sure the schema itself is versioned: #139

[domdomegg]

Note: we should be thoughtful about how to define version as per https://github.com/modelcontextprotocol/registry/pull/134/files#r2145287494

Discussion continuing on this in #158