Improve README.md for Medalino CMS

[coldhell7]

I have completely overhauled the root README.md to provide a professional and comprehensive overview of the Medalino CMS project. The new README includes:

• A clear description of the project's purpose and value.
• A list of key features (AI content, SEO, Performance).
• A detailed tech stack overview (Next.js 16, Astro 6, Tailwind 4, Drizzle, etc.).
• A map of the monorepo structure.
• Updated and accurate commands for installation, development, and database management.
• Documentation for all necessary environment variables.
• Deployment guidance for Nginx and systemd.

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PR created automatically by Jules for task 7697845517917996129 started by @coldhell7

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