Add $schema to server.json specification to enable versioning, IDE autocomplete (#308)

## Summary
- Add optional `$schema` property to ServerDetail specification
- Update JSON Schema, OpenAPI spec, Go models, examples, and publisher
template
- Enable schema versioning and IDE validation support

## Changes
- Added `$schema` as optional string property with URI format in specs
- Updated Go ServerDetail struct with new Schema field
- Added `$schema` to examples and publisher template
- Copy schema file to ../static for GitHub Pages hosting

Fixes #139

docs/server-json/registry-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://modelcontextprotocol.io/schemas/draft/2025-07-09/registry-server.json",
   "title": "MCP Server Detail - Registry Schema",
   "description": "Registry-specific constraints for MCP server representation. Extends the base schema with additional validation rules.",
-  "$ref": "https://modelcontextprotocol.io/schemas/draft/2025-07-09/server.json",
+  "$ref": "https://static.modelcontextprotocol.io/schemas/2025-07-09/server.schema.json",
   "properties": {
     "repository": {
       "properties": {