[docs] Document release publishing workflow (#73)

[coisa]

Summary

• Add a maintainer guide for release and publishing flow.
• Document pre-release checks, tag and Packagist expectations, Pages and wiki publication, and post-release validation.
• Keep current manual release steps distinct from future automation opportunities.

Verification

• git diff --check
• composer dev-tools docs

Closes #73

[github-actions]

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• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-80/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-80/coverage/