Update README with comprehensive folder documentation and individual README files

[Copilot]

This PR addresses the need to document all folders in the repository by updating the main README.md and adding individual README files for folders that were missing documentation.

Changes Made

Main README.md Enhancement

• Transformed the basic README into a comprehensive overview of all Azure Verified Modules examples
• Added structured documentation for all four example folders with clear descriptions
• Included getting started guide, prerequisites, and customization information
• Applied consistent formatting with emojis and organized sections

New Individual README Files

Created detailed documentation for folders that were missing README files:

ase/README.md - Documents the App Service Environment v3 example including:

• Complete network infrastructure setup
• Virtual network with dedicated subnets
• Application Gateway and Bastion Host configuration
• Windows and Linux app service plans
• Sample web applications with deployment slots

managedrunners/README.md - Documents the GitHub managed runners example including:

• User Assigned Managed Identity with federated credentials
• Key Vault integration with private endpoint access
• Secure network architecture for GitHub Actions workflows
• Cross-region network peering capabilities

Existing Documentation

Preserved existing README files for dns/ and starhubspoke/ folders which already had proper documentation.

Result

All four example folders now have comprehensive documentation that helps users understand:

• The purpose and use case of each example
• Key components and architecture patterns
• How to deploy and customize the solutions

This makes the repository much more accessible for users looking to learn from and implement these Azure Verified Modules examples.

Fixes #25.

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