Create experimental extension registry

[wbreza]

Description

Stand up a new registry dedicated to experimental extensions, published under a new aka.ms link for easy access. This registry will host bleeding-edge extensions that aren't ready for the main registry.

Acceptance Criteria

Registry Setup

• Create new registry JSON file for experimental extensions
• Host registry in appropriate GitHub repository/branch
• Set up aka.ms link (e.g., aka.ms/azd/registry/experimental)
• Configure registry metadata and schema

Registry Guidelines

• Define criteria for experimental vs. main registry
• Document stability expectations for experimental extensions
• Establish process for promoting experimental → main
• Create submission guidelines for experimental extensions

User Documentation

• Document how to add experimental registry to azd
• Set clear expectations about stability and support
• Provide examples of adding and using experimental extensions
• Include troubleshooting guidance

Integration

• Update azd to recognize experimental registry
• Test registry resolution with multiple registries active
• Ensure proper priority/ordering with other registries

Benefits

• Provides sandbox for testing new extensions
• Reduces barrier for community contributions
• Protects main registry stability
• Enables faster iteration on experimental features

Parent Issue

• [EPIC] Extension enhancements #5767 (Extension enhancements EPIC)