Fix misleading server.json schema URL label (#803)

The schema in this repo may contain unreleased changes, so labeling it
with a dated version URL is misleading. Change to use a `draft` URL
instead, and add documentation for the schema contribution process.

- Update $id in server.schema.json to use draft URL
- Update example $schema URL in openapi.yaml
- Add Draft (Unreleased) section to CHANGELOG.md
- Add CONTRIBUTING.md with process for making/releasing schema changes

Written by Claude Code, reviewed and tweaked by me.

Co-authored-by: Claude <noreply@anthropic.com>

docs/reference/api/openapi.yaml

@@ -709,7 +709,7 @@ components:
           type: string
           format: uri
           description: JSON Schema URI for this server.json format
-          example: "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json"
+          example: "https://static.modelcontextprotocol.io/schemas/draft/server.schema.json"
         packages:
           type: array
           items: