Make cargo-dist-json-schema.json useful

* We should add a `"$schema": "https://..."` key to dist-manifest.json
* We should store a versioned copy of the schema somewhere more stable
* Teach cargo-dist to have adhoc static artifacts and just upload it to releases?
* This may have wonky behaviour for prereleases which might link the wrong/non-existent versions

Start by locating cargo-dist-json-schema.json and the code that generates dist-manifest.json, then trace how cargo-dist publishes release artifacts. Compare the requested $schema reference with the versioned-copy and static-artifact options, including prerelease behavior; done means the schema is stable, discoverable, and correctly linked for releases.

Written by the indexing model from the issue text.