docs: Add documentation for PROJECT_DIR environment variable

[coderabbitai]

Context

The PROJECT_DIR environment variable is used in docker-compose.yaml for critical paths in multiple services but lacks proper documentation.

Required Documentation

1. PROJECT_DIR environment variable:

   • Required structure and permissions
   • Validation requirements
   • Example configuration in .env file

2. Setup instructions:

   • Directory structure requirements
   • File permissions
   • Security considerations for container file access

References

• PR: chore(wren-ai-service): minor updates #1236
• Discussion: chore(wren-ai-service): minor updates #1236 (comment)

Assignee

@wwwy3y3