Add drizzle-kit announcement manifest and schema doc

The manifest ships with the kill switch off and one pre-seeded entry
nudging 0.x installs toward the rc tag, range-fenced as
`>=0.31.11 <1.0.0-0` so prerelease-inclusive matching can never fire
it on the rc line. The README documents the schema, range authoring
on both bounds, and the CDN update/purge runbook.