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/CHANGELOG.md

@@ -2,6 +2,20 @@
 
 Changes to the server.json schema and format.
 
+## Draft (Unreleased)
+
+This section tracks changes that are in development and not yet released. The draft schema is available at [`server.schema.json`](./server.schema.json) in this repository.
+
+### Changed
+
+- No changes yet.
+
+### Notes
+
+When ready for release, changes in this section will be moved to a dated version section (e.g., `## 2025-XX-XX`) and the schema will be published to a versioned URL.
+
+---
+
 ## 2025-10-17
 
 ### Changed