Add GitHub Actions documentation for quickstart image

[Copilot]

Adds comprehensive documentation for the GitHub Action introduced in PR #671, making it discoverable for developers who want to use the quickstart image in their CI/CD workflows.

What's Added

• New "Using in GitHub Actions" section in README.md with:
  • Supported platforms (Linux, macOS Intel) and limitations
  • Basic usage example
  • Complete configuration options reference
  • Advanced health check settings
  • Real-world workflow example for smart contract testing

Usage Examples

Basic usage:

- name: Run Stellar quickstart
  uses: stellar/quickstart@main

With configuration:

- name: Run Stellar quickstart
  uses: stellar/quickstart@main
  with:
    network: "local"
    enable: "core,horizon,rpc"

Full workflow example for testing:

- name: Start Stellar network
  uses: stellar/quickstart@main
  with:
    network: "local"
    enable: "core,horizon,rpc"

- name: Run contract tests
  run: |
    # Services available at:
    # - Horizon: http://localhost:8000
    # - RPC: http://localhost:8000/rpc  
    # - Friendbot: http://localhost:8000/friendbot

The documentation covers all available inputs and provides clear guidance on platform support, making the action easily discoverable and usable.

Fixes #684.

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