feat: database setup instructions to oss deployment docs

[Shiti]

Summary

Introduce a section for configuring a PostgreSQL database for Logto, including user creation, permissions, and schema setup. This addition also highlights using a batch job for schema initialization and specifies best practices for Docker image versioning and permissions.

[Copilot]

Pull Request Overview

This PR adds comprehensive database setup instructions to the Logto OSS deployment documentation, providing users with guidance on configuring PostgreSQL for production deployments.

• Introduces PostgreSQL user creation and permission configuration steps
• Provides a Kubernetes batch job example for database schema initialization
• Includes best practices for Docker image versioning and user permissions

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