Proposal: store docs.json in the registry

As part of #525 should we consider storing docs.json in the registry?

• The registry pipeline is already compiling projects with an appropriate compiler. We can easily add the appropriate codegen targets.
• The registry pipeline is the sole publisher to Pursuit nowadays. It seems unnecessary to support additional authentication workflows in Pursuit.
• If we store docs metadata in the registry, pursuit largely becomes a stateless registry client in principle, even though it likely needs to build it's own index.
• We have more freedom to adjust the actual format (especially wrt metadata) since the registry controls the data end-to-end.

Questions:

• Where is this actually stored? Package metadata is stored in a single-file ldjson format, but we would want to store version specific files, likely.
• Are there format changes we need to make? Is there redundant information in the data we currently publish to Pursuit that is otherwise available in the registry?
• We may need a data migration from pursuit-backups.
• We may need some additional postback mechanism to notify Pursuit of newly published packages if we don't want to rely on a cron job that pulls the registry metadata.