Organize test configuration and repository documentation

[MortenFriisSiteImprove]

Summary

Group test configuration and documentation under tests/ so the repository root is easier to navigate.

• Move all four Playwright configurations into tests/config/ and the testing guide to tests/README.md.
• Update npm commands and the live-fixture safety test reference. Resolve test, result and report paths from the repository root so existing artifacts and commands keep working.
• Consolidate the old release guide into docs/workflows.md, retaining preparation and troubleshooting details and removing obsolete instructions.
• Update documentation links and remove the obsolete root Playwright exclusion from PHP_CodeSniffer.

No plugin code, workflow triggers, test assertions or release behavior changes.

Validation

• Browser suite: 42 passed in Chromium and Firefox.
• Live-runner contracts: 14 passed; regression catalog references pass.
• Test discovery: browser 42, WordPress 18, multisite 32, live fixtures 5.
• Compared all four configs before/after: test directories, result directories and browser projects are unchanged.
• Relative documentation links and diff whitespace checks pass.
• GitHub PR checks will run the WordPress and multisite suites using the relocated configurations.