[SirDarcanos]

Summary

Adds a contributing guide written for people: local setup, reporting bugs, safe offline tests, documentation updates and pull requests against main. Coding assistants are directed to read AGENTS.md as well.

README now links to the guide instead of duplicating development instructions. The existing internal link to those instructions follows the move. The repository link checker also scans CONTRIBUTING.md, so CI checks its outgoing local links.

Verification

• All 498 local Markdown links across 52 files pass.
• PHP syntax and Git whitespace checks pass.
• An isolated regression check accepted a valid contributing-guide link and rejected a broken one.
• The documented Composer test-command forwarding was checked with PHPUnit help under network denial. The application test suite was not rerun for this documentation change.

No library implementation, dependencies, PHPUnit tests or CI job definitions change. Existing documentation CI builds this PR; merging triggers the already-configured documentation publication. CI duration/compute impact is unmeasured.

Documentation-only maintenance: no release-impact label, changelog entry or package release, as requested.