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/server-json/server.schema.json

@@ -1,6 +1,6 @@
 {
   "$comment": "This file is auto-generated from docs/reference/api/openapi.yaml. Do not edit manually. Run 'make generate-schema' to update.",
-  "$id": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+  "$id": "https://static.modelcontextprotocol.io/schemas/draft/server.schema.json",
   "$ref": "#/definitions/ServerDetail",
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
@@ -373,7 +373,7 @@
       "properties": {
         "$schema": {
           "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",
           "format": "uri",
           "type": "string"
         },