Fix misleading server.json schema URL label

[tadasant]

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.