Write Elementor to Core blocks migration guide

[zahidgh]

Summary

• replace the Elementor migration placeholder with a complete page-by-page workflow
• cover backups, staging, dependency inventory, design systems, patterns, templates, and launch rollback
• map common Elementor widgets and Theme Builder concepts to Core blocks and Site Editor tools
• add focused QA guidance for responsive layouts, accessibility, forms, WooCommerce, SEO, analytics, and performance
• include troubleshooting for unsupported widgets, dynamic data, form delivery, and remaining Elementor dependencies

Validation

• markdownlint-cli2 guides/elementor-to-core-blocks.md
• markdown-link-check guides/elementor-to-core-blocks.md
• git diff --check
• checked referenced behavior against current WordPress Site Editor, Styles, Patterns, and Elementor export documentation

Closes #42