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.

[domdomegg]

Do we want people to use 'draft' version? Also given https://static.modelcontextprotocol.io/schemas/draft/server.schema.json doesn't exist I worry this might cause problems if people end up using this etc.

[tadasant]

Do we want people to use 'draft' version? Also given https://static.modelcontextprotocol.io/schemas/draft/server.schema.json doesn't exist I worry this might cause problems if people end up using this etc.

I don't think we generally want people to use the draft version -- they should use one (ideally the latest one) we've finalized in static.

Fair point that it's confusing that it doesn't exist at that URL. Maybe would be worth pointing it more directly at the current-draft. But I think we can take that as a follow-on to this; more critical IMO to make it clear the version in this repo is not exactly the same as the latest one in static.

Edit: follow up Issue: #814