docs: advance v3 release baseline

[aatuh]

Summary

• remove the temporary .next_steps.md release checklist after v3.1.0 publication
• advance release baseline examples and release workflow defaults from v3.0.2 to v3.1.0
• update contrib to depend on the published root v3.1.0 module and keep docscheck contracts aligned

Validation

• GOWORK=off GOTOOLCHAIN=local make docs-check
• GOWORK=off GOTOOLCHAIN=local make finalize
• go list -m github.com/aatuh/api-toolkit/contrib/v3@v3.1.0

Release notes

• Published v3.1.0 root release before this cleanup.
• Published contrib/v3.1.0 tag at the same release commit so contrib module installs resolve.

[aatuh]

Owner approval recorded from maintainer request in Codex: remove the temporary .next_steps.md file and complete the post-v3.1.0 baseline cleanup.

All PR checks are passing. Because this repository has a single code owner and GitHub does not count self-approval toward the required review, I am using an admin merge for this cleanup PR after recording this approval context.