docs: say the default squash body drops footers; correct the #54 example (#61)

[abienkowski]

Description

#60 was merged with this repo's default squash body (COMMIT_MESSAGES, the commit list). Its intended Release-As: v0.3.0 footer therefore never reached main. The new version step worked correctly on its first real run (Latest tag: v0.2.26; range: v0.2.26..HEAD, bump=patch, tag=v0.2.27, 11 assets) and shipped v0.2.27.

This PR updates docs/release-versioning.md:

• Squash guidance: the default squash body leaves out footers written in the PR description, so add them at merge time, for example with gh pr merge --squash --subject … --body-file …. Each footer line must start at column 0.
• Worked examples: Release pipeline always bumps patch; AGENTS.md claims versions derive from commit types #54 shipped as v0.2.27 (patch, correct for Release pipeline always bumps patch; AGENTS.md claims versions derive from commit types #54 alone), and v0.3.0 is cut by Release docs: default squash body drops Release-As/BREAKING CHANGE footers; cut v0.3.0 #61.

Closes #61

Merging this PR cuts v0.3.0

Dry run of the exact pipeline on a simulated squash of this branch onto main:

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

Squash-merge with the message file (not the default body, and not rebase):

gh pr merge <this PR> --squash \
  --subject "docs: say the default squash body drops footers; correct the #54 example (#61)" \
  --body-file squash-61.txt

squash-61.txt:

Document that the repo default squash body leaves out PR-description
footers, and correct the #54 worked example.

Release-As: v0.3.0

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Implementation(s) changed

• Go
• Rust
• TypeScript
• Quint specification
• CI / infrastructure

Testing

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

Checklist

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