why: Cut alpha.17 from master, so the release's tag and attestations
name a commit on trunk. alpha.16 shipped the same code from the pull
request branch before it merged.

what:
- Date the alpha.17 changelog section, stating that no user-visible
  behaviour changed since alpha.16
- Move the install snippets to alpha.17
- Advance the development version to alpha.17-SNAPSHOT
- Set the API baseline to the published alpha.16 release, recording its
  artifacts' checksums and dropping alpha.15's