Document extension promotion workflow (dev/experimental → main registry)

## Description

Create comprehensive documentation explaining how an extension moves from dev or experimental registries to the main registry, including steps for authors and what users can expect during the transition.

## Acceptance Criteria

### Author Documentation

- [ ] Document promotion criteria (quality, testing, stability requirements)
- [ ] Outline step-by-step promotion process
- [ ] Explain required testing and validation
- [ ] Describe versioning requirements for promotion
- [ ] Define communication expectations (changelog, release notes)

### User Documentation

- [ ] Explain what happens to installed extensions during promotion
- [ ] Document automatic upgrade behavior (if applicable)
- [ ] Clarify version continuity and breaking changes
- [ ] Provide migration guidance for users

### Process Documentation

- [ ] Define who approves promotions
- [ ] Establish timeline expectations
- [ ] Document rollback procedures if issues arise
- [ ] Create promotion checklist template

### Communication Plan

- [ ] Define announcement process for promotions
- [ ] Template for promotion notifications
- [ ] Guidance on deprecating dev/experimental versions
- [ ] User communication during transition period

## Benefits

- Clear expectations for extension authors
- Transparency for extension users
- Consistent promotion process
- Reduced confusion during transitions

## Parent Issue

- #5767 (Extension enhancements EPIC)