Add documentation for Plugin Domains in UltimateAuth

Introduced documentation for Plugin Domains in UltimateAuth, detailing architecture, extensibility, and recommended approaches.

Author: mckaragoz

docs/content/plugin-domains/index.md

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+# 🧩 Plugin Domains
+
+Authentication alone is not enough.
+
+Real-world systems also require:
+
+- User management  
+- Credential handling  
+- Authorization rules  
+
+👉 UltimateAuth provides these as **plugin domains**
+
+## 🧠 What Is a Plugin Domain?
+
+A plugin domain is a **modular business layer** built on top of UltimateAuth.
+
+👉 Core handles authentication  
+👉 Plugin domains handle identity and access logic  
+
+<br>
+
+## 🏗 Architecture
+
+Each plugin domain is composed of multiple layers:
+
+### 🔹 Bridge Package
+
+Defines the server needed bridge interfaces:
+
+👉 This package provides required minimal info for server package.
+
+<br>
+
+### 🔹 Contracts Package
+
+Shared models between:
+
+- Server  
+- Client  
+
+👉 Includes DTOs, requests, responses
+
+### 🔹 Reference Implementation
+
+Provides default behavior:
+
+- Application services  
+- Store interfaces  
+- Default implementations  
+
+👉 Acts as a production-ready baseline
+
+### 🔹 Persistence Layer
+
+Provides storage implementations:
+
+- InMemory  
+- Entity Framework Core
+
+👉 Additional providers (Redis, etc.) can be added
+
+<br>
+
+## 🔄 Extensibility Model
+
+Plugin domains are designed to be:
+
+- Replaceable  
+- Extendable  
+- Composable  
+
+👉 You can implement your own persistence  
+👉 You can extend behavior
+
+<br>
+
+## ⚠️ Recommended Approach
+
+In most cases:
+
+👉 You should NOT replace a plugin domain entirely  
+
+Instead:
+
+- Use provided implementations  
+- Extend via interfaces  
+- Customize behavior where needed  
+
+👉 This ensures compatibility with the framework
+
+<br>
+
+## 🧠 Mental Model
+
+If you remember one thing:
+
+👉 Core = authentication engine  
+👉 Plugin domains = business logic  
+
+## 🎯 Why This Matters
+
+This architecture allows UltimateAuth to:
+
+- Stay modular  
+- Support multiple domains  
+- Enable enterprise customization  
+- Avoid monolithic identity systems  
+
+👉 You don’t build everything from scratch  
+👉 You assemble what you need
+
+## ➡️ Next Step
+
+- Manage users → Users
+- Handle credentials → Credentials
+- Control access → Authorization