Add community health files

[goitsemangmajaga]

Summary

• Add contributing guidelines and a contributor code of conduct
• Add a pull request template
• Add YAML issue forms for bugs, features, and documentation
• Disable blank issues and add community contact links

Closes #14

Type of change

• Bug fix
• New feature
• Documentation update
• Refactoring
• Tests
• Other

Verification

• Confirmed all three issue forms render correctly on the fork
• Confirmed the required title prefixes and fields appear
• Confirmed blank issues are restricted to maintainers
• Confirmed the forum, documentation, and security links appear

Checklist

• I have read CONTRIBUTING.md
• vendor/bin/phpunit passes
• vendor/bin/php-cs-fixer fix --dry-run --diff reports no issues
• Tests have been added or updated where appropriate
• Documentation has been updated where appropriate
• Backwards compatibility has been preserved

The test and PHP CS Fixer commands were not run because this pull request adds
community documentation and YAML configuration only. No PHP files were changed.

Backwards compatibility notes

No public APIs or application behaviour are changed.