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-registry-api/openapi.yaml

@@ -450,6 +450,11 @@ components:
         - $ref: '#/components/schemas/Server'
         - type: object
           properties:
+            $schema:
+              type: string
+              format: uri
+              description: JSON Schema URI for this server.json format
+              example: "https://static.modelcontextprotocol.io/schemas/2025-07-09/server.schema.json"
             packages:
               type: array
               items: