Make it easier to create and manage custom extension registries

## Description

Update the `azd x` (developer extension) to help users create and manage custom extension registries that will be hosted on GitHub, empowering teams to build and share their own curated extension sets.

## Acceptance Criteria

### Registry Creation

- [ ] Add `azd x registry create` command to scaffold new registry
- [ ] Generate registry JSON schema with proper structure
- [ ] Create GitHub repository template or initialization
- [ ] Include README with setup instructions

### Registry Management

- [ ] Add `azd x registry add-extension` command to add extensions to registry
- [ ] Validate extension manifests before adding
- [ ] Support versioning for registry entries
- [ ] Generate or update registry metadata

### Automation

- [ ] Include GitHub Actions workflow for publishing
- [ ] Automate validation of registry schema
- [ ] Support automated version bumps
- [ ] Provide CI/CD templates

### Documentation

- [ ] Create guide for setting up custom registry
- [ ] Document registry hosting on GitHub
- [ ] Provide examples of registry configurations
- [ ] Include best practices for registry management

## Benefits

- Empowers teams to build internal extension catalogs
- Enables organizations to share curated extension sets
- Reduces friction for custom registry setup
- Provides automation for registry maintenance

## Parent Issue

- #5767 (Extension enhancements EPIC)