Add WP-CLI WordPress migration guide

[zahidgh]

Summary

• add a repeatable WP-CLI workflow for WordPress-to-WordPress content migrations
• document both WXR content imports and complete database/filesystem migrations
• cover backups, URL replacement, serialized data, multisite cautions, derived data, verification, cutover, rollback, and troubleshooting
• link each major command to the official WP-CLI reference

Validation

• git diff --check
• reviewed command syntax against the official WP-CLI documentation

Closes #65