feat(config): add .env.example file for apps/sim

[ManvithGopu13]

Description

This PR adds a comprehensive .env.example file to apps/sim to help developers quickly set up their local environment.

Changes

• Added .env.example with 382 lines covering all environment variables
• Organized into logical sections with clear comments
• Marked required vs optional variables
• Included default values and generation instructions where applicable
• Added descriptions for all variables

Sections Covered

• Core Database & Authentication
• Copilot AI Configuration
• Payment & Billing (Stripe)
• Email & SMS Communication
• AI/LLM Provider API Keys
• Cloud Storage (AWS S3, Azure Blob)
• OAuth Integrations
• SSO Configuration (OIDC & SAML)
• Monitoring & Analytics
• Rate Limiting & Data Retention
• UI Branding & Whitelabeling
• Feature Flags

Related Issue

Fixes #1835

Testing

• ✅ File created successfully in apps/sim/
• ✅ All environment variables from lib/env.ts included
• ✅ Follows the structure mentioned in README.md
• ✅ Ready for developers to copy to .env and configure

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