Add documentation for GitHub Actions working-directory configuration in multi-project solutions

[Copilot]

Adds comprehensive documentation to address scenarios where GitHub Actions workflows generated by azd pipeline config need manual configuration for multi-project .NET Aspire solutions.

Changes Made

Added a new section "Configure working directory for multi-project solutions" to the GitHub Actions deployment documentation that explains:

1. When working-directory is needed: Multi-project solutions where the AppHost project is not in the repository root
2. How to identify the issue: Generated workflows assume AppHost is in root directory
3. Step-by-step solution: How to add working-directory parameters to workflow steps
4. Before/after examples: Clear YAML examples showing the required changes
5. Finding the correct path: How to identify which directory contains the AppHost project

Example scenario covered

For a repository structure like:

MyAspireApp/
├── MyAspireApp.sln
├── MyAspireApp.AppHost/
│   ├── MyAspireApp.AppHost.csproj
│   └── Program.cs
├── MyAspireApp.ApiService/
└── MyAspireApp.Web/

The documentation now shows how to update workflow steps:

- name: Provision Infrastructure
  run: azd provision --no-prompt
  working-directory: ./MyAspireApp.AppHost  # Added this line
  env:
    AZURE_ENV_NAME: ${{ vars.AZURE_ENV_NAME }}
    # ... other env vars

Documentation quality

• Follows Microsoft Writing Style Guide conventions
• Passes all markdown linting checks
• Includes cross-reference to existing "Add .NET Aspire to existing app" tutorial
• Uses clear, actionable language with step-by-step instructions

Fixes #1696.

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Internal previews

📄 File	🔗 Preview link
docs/deployment/azure/aca-deployment-github-actions.md	docs/deployment/azure/aca-deployment-github-actions

[adegeo]

Approved to unblock. Added some feedback.