[STORY] v4-S28: Write v3 to v4 migration guide

[raifdmueller]

User Story

As a documentation author, I want a clear migration guide explaining what changed between v3 and v4 and what I need to do.

Content

• What was removed (Gradle, core module, jBake)
• What replaced it (direct Groovy execution, scripts, custom SSG)
• Config compatibility (nothing to change)
• Command changes (if any)
• Known breaking changes
• FAQ

Acceptance Criteria

```gherkin
Given the migration guide
When a v3 user reads it
Then they understand all changes in under 10 minutes
And they can upgrade their project successfully
```

Priority: Must

Epic: v4-E5 (#7)

🤖 Generated with Claude Code

[raifdmueller]

Analysis

Migration Guide Structure

1. What changed — Gradle removed, core module dissolved, jBake replaced, AsciiDoctor externalized (1 page)
2. What you need to do — Usually nothing (config is compatible). If you used ./gradlew directly, switch to ./dtcw. (half page)
3. New requirements — AsciiDoctor CLI must be installed (auto-installed by dtcw, ADR-6) (half page)
4. Custom task migration — If you had custom Gradle tasks, convert to Groovy scripts (1 page with example)
5. FAQ — Common error messages and solutions (1 page)

Format

AsciiDoc, published as part of the microsite under the development section.

Key Message

For most users: nothing changes. ./dtcw generateHTML works as before. Config files are compatible. The migration guide should reassure, not alarm.

🤖 Generated with Claude Code