Add WPBakery to Core blocks migration guide

[zahidgh]

Summary

• replace the WPBakery migration placeholder with a page-by-page conversion workflow
• map common WPBakery elements to Core blocks and explain how to handle addon shortcodes
• cover templates, patterns, responsive layouts, forms, WooCommerce, custom fields, SEO, analytics, and accessibility
• add staging, QA, cutover, rollback, and troubleshooting guidance

Validation

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

Closes #45