docs: put release footers in the commit itself; cut v0.3.0 (#63)

[abienkowski]

Description

#64 was merged with an empty squash body, so its Release-As: v0.3.0 footer missed main, as it had for #60 and #62. Release run 37774150988 was cancelled before tagging. The cause: the footer only ever existed in a separate merge-message file. GitHub's default squash message for a single-commit PR is the commit's own message, and that commit had no body.

This PR's single commit carries the footer in its own message, so a default "Squash and merge" (web or CLI, no custom message) keeps it. docs/release-versioning.md now recommends this, and its worked examples record #64.

Closes #63

Dry run (simulated default squash = commit message + (#N))

latest=v0.2.27
bump=release-as
tag=v0.3.0

Merge: plain "Squash and merge" with the default message. Don't rebase, and don't replace the body.

Type of change

• Documentation update

Testing

• Dry run of scripts/release-version.sh on a simulated default squash (above)
• Unit / integration: not run, because this PR changes docs only

Checklist

• I have read CONTRIBUTING.md
• I have updated documentation as needed