docs: Improve Extension Development Documentation

[imtuyethan]

Problem Statement

https://discord.com/channels/1107178041848909847/1326294926622986282

Developers attempting to create extensions for Jan are facing challenges understanding:

• The core tech stack (Electron + Next.js)
• How to properly implement node.js libraries and handle server-side vs client-side code
• The model provider system and extension integration
• Proper redirect URL configuration for OAuth implementations
• Deep linking and URL handling in the Electron environment

Feature Idea

Create comprehensive extension development documentation that includes:

• Clear overview of Jan's tech stack architecture (Electron + Next.js)
• Guide for handling OAuth and authentication in extensions
• Best practices for using node.js libraries in the Electron context
• Tutorial on implementing model providers
• Examples of common extension patterns
• Troubleshooting section addressing common errors (like the require issue mentioned)
• Documentation on app deep linking and URL handling

The documentation should be beginner-friendly and include practical examples, as we're seeing developers struggle with fundamental concepts when trying to create extensions.

The primary goal is to reduce confusion and enable developers to successfully create extensions without running into basic architectural misunderstandings.