Clarify approach to managing server.json schema versions

I think the state we want to be in is:

• Registry code (including API and CLI) uses exclusively finalized versions of server.json from https://github.com/modelcontextprotocol/static/tree/main/schemas
• This file is a draft that should not be used by any systems (neither registry code nor others) until it is finalized and in the static repo
• After a server.json version release to static, we should always cut a ticket to update the registry code (API and CLI) to use the new version

If agreed, we should:

1. Change that server.schema.json $id field to point to draft
2. Document the sequencing for handling server.schema.json changes in accordance with the above

Are there any dependencies between our code and that server.schema.json field right now? If so, I think we should sever them and have the API/CLI code reference the static versions.

See: https://discord.com/channels/1358869848138059966/1369487942862504016/1424904131331625031

Start by checking whether the registry API or CLI references docs/reference/server-json/server.schema.json, then compare those references with finalized versions in the static repository. Confirm the draft $id and document the sequencing for schema changes. Done means the API and CLI use finalized static versions and the update process is recorded.