Simplify GitHub Actions and document testing and releases

[MortenFriisSiteImprove]

Summary

Consolidate the overlapping Actions entry points into four named workflows: Code standards, Plugin tests, Live Siteimprove test, and Create and deploy release, plus the shared release-check helper.

• Move manual plugin-ref and WordPress/PHP selection into Plugin tests, retaining browser, single-site and multisite coverage and existing required-check names.
• Remove the three duplicate manual test workflows and the separate Marketplace deployment workflow.
• Keep one publishing route with explicit dry-run/test/production modes. Require version tags; remove the old main and wp-* deployment triggers. Dry runs create no GitHub release and perform no SVN operations. Test mode requires a separate SVN destination. Serialize release runs.
• Add docs/workflows.md, link it from README, and replace obsolete deployment instructions with the current procedures, report locations and live-test limitations.

Validation

• All five workflow YAML files parse; embedded shell syntax and local reusable-workflow references pass.
• Thirteen release-routing cases pass, including rejection of branch refs, malformed tags and production requests on test/dry-run tags.
• Live-runner contracts: 14 passed. Regression catalog references and diff whitespace checks pass.
• PR checks exercise the consolidated plugin checkout in GitHub. No live account test or deployment was triggered for this cleanup.

Release behavior

Plain vMAJOR.MINOR.PATCH tag pushes still validate and publish automatically. -dry-run and -test suffixes select their respective modes; manual runs select an existing version tag and default to dry-run. All modes retain the shared validation, including the protected live check. The live missing-title result assertion remains skipped and is documented explicitly.