docs(dev): add local PostgreSQL setup examples

[ArshSSandhu]

Summary

This PR clarifies the local Docker Compose setup for new contributors by adding example PostgreSQL .env values and common troubleshooting notes.

Why

While setting up the project locally, Docker Compose can fail if required PostgreSQL environment variables, such as POSTGRES_PASSWORD, are left blank after copying example.env to .env.

This update makes the local development setup easier to follow, especially for contributors setting up the project for the first time.

Changes

• Added example local PostgreSQL .env values for Docker-based development
• Clarified that SQLite and MySQL/MariaDB variables can remain blank when using PostgreSQL
• Added troubleshooting notes for missing POSTGRES_PASSWORD
• Added Windows Docker Desktop troubleshooting for the dockerDesktopLinuxEngine error

Testing

• Copied example.env to .env
• Added local PostgreSQL values
• Ran docker compose up -d
• Verified Docker Desktop was running before retrying Docker Compose
• Previewed the Markdown documentation locally