Add package-level CONTRIBUTING guides for publishing directories

[Copilot]

Add package-local contribution guidance under each top-level packages/ directory. These docs make it explicit that the package directories are publishing targets, not contribution entry points, and redirect PRs to microsoft/kiota-php.

• What changed

  • Added CONTRIBUTING.md to:
    • packages/abstractions
    • packages/authentication
    • packages/bundle
    • packages/http
    • packages/serialization

• Guidance added

  • Clarifies that each package directory is only used for publishing
  • States that pull requests should not target the package directory
  • Redirects contributions and PRs to microsoft/kiota-php

• Consistency

  • Uses the same concise wording in each package directory
  • Leaves the root CONTRIBUTING.md unchanged

# Contributing

This package directory is only used for publishing.

Do not open pull requests against this package directory. Open contributions and pull requests in [microsoft/kiota-php](https://github.com/microsoft/kiota-php) instead.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud