Add migration decision guide and social previews

[Maksim-Burtsev]

What changed

• add an honest, indexable decision guide comparing PyClickHouseMigrator with raw scripts, Goose, Flyway, Liquibase, and clickhouse-migrations
• document fit, alternatives, operational boundaries, and a production deployment flow
• add the guide to navigation, the homepage, README, llms.txt, and llms-full.txt
• add responsive comparison cards for mobile
• add site-wide Open Graph and Twitter metadata
• add the approved 1280x640 social preview asset

Why

This gives developers, search engines, and coding agents one source-backed page for deciding whether the project fits a ClickHouse migration workflow, while improving link previews across external surfaces.

Validation

• uv run --frozen --group docs zensical build --strict --clean
• every generated sitemap route returned HTTP 200 locally
• every external comparison source returned HTTP 200
• browser QA at desktop and mobile widths
• light and dark themes
• no horizontal overflow or browser console warnings/errors
• pre-commit hooks passed