docs(readme): enhance documentation for apter-tech/ssh-agent action

[warnyul]

• Expanded README to include detailed usage examples for integrating apter-tech/ssh-agent into GitHub workflows.
• Added sections for:
  • Features: Highlights SSH agent integration, host verification, key type filtering, and post-job cleanup.
  • Usage: Demonstrates basic and advanced configurations, including predefined known hosts.
  • Inputs: Lists new and inherited inputs with detailed descriptions.
  • Security Considerations: Emphasizes host key verification practices.
  • Development: Provides an overview of core scripts for contributors.
  • Included a license note for clarity on usage rights.

This update provides comprehensive guidance for users and contributors, improving clarity and ease of use.