docs: clarify staged publishing can create new packages

[shmam]

Summary

Clarifies the staging documentation to make explicit that staged publishing can now be used to create brand-new packages, not just to stage new versions of packages that already exist.

Changes

In api/registry.npmjs.com/stage.yaml:

• POST /-/stage/package/{package-name}
  • Updated the summary to mention that the package is created if it does not yet exist.
  • Added a paragraph explaining that when a package does not yet exist, staging its first version creates the package on approval — no need to publish it directly beforehand — provided the publisher is permitted to create the name.
  • Reworded the requirement bullet from the ambiguous "Package MUST exist or be creatable by the authenticated publisher" to spell out that staging the first version creates a new package on approval.
• POST /-/stage/{stage-id}/approve
  • Noted that approving the first staged version of a non-existent package creates the new package in the registry.

Validation

• npm run lint:yaml passes (Your API description is valid. 🎉). The remaining warnings are pre-existing and unrelated to these text-only edits.

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