Add Divi to Core blocks migration guide

[zahidgh]

Summary

• replace the Divi migration placeholder with a staged Divi Builder to Core blocks workflow
• map Divi sections, rows, modules, global modules, Theme Builder templates, and responsive settings
• cover dynamic content, forms, WooCommerce, SEO, analytics, accessibility, QA, cutover, rollback, and troubleshooting

Validation

• git diff --check
• reviewed the guide against the repository's existing migration-guide structure

Closes #43