Separate out Registry-specific `server.json` out from generic `server.json`

[tadasant]

Analogous to #149, we should have a (more constrained) schema for the official Registry vs. a generic server.json that will serve other use cases.

See comment: https://github.com/modelcontextprotocol/registry/pull/153/files/8041f5f359d241862a5de1ddc8574fe79c4c44b0#r2172966211

Related: #153