Add MkDocs documentation portal with Material theme and CI/CD workflow

Co-authored-by: Fu-Jie <33599649+Fu-Jie@users.noreply.github.com>

docs/development/index.md

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+# Development
+
+Learn how to develop plugins and contribute to OpenWebUI Extras.
+
+---
+
+## Getting Started
+
+<div class="grid cards" markdown>
+
+-   :material-book-open-page-variant:{ .lg .middle } **Plugin Development Guide**
+
+    ---
+
+    The comprehensive guide covering everything from getting started to advanced patterns and best practices.
+
+    [:octicons-arrow-right-24: Read the Guide](plugin-guide.md)
+
+-   :material-github:{ .lg .middle } **Contributing**
+
+    ---
+
+    Learn how to contribute plugins, prompts, and documentation to the project.
+
+    [:octicons-arrow-right-24: Contribution Guide](../contributing.md)
+
+</div>
+
+---
+
+## Plugin Types Overview
+
+OpenWebUI supports three main plugin types:
+
+| Type | Purpose | Entry Method |
+|------|---------|--------------|
+| **Action** | Add buttons to messages | `action()` |
+| **Filter** | Process messages | `inlet()` / `outlet()` |
+| **Pipe** | Custom model integration | `pipe()` |
+
+---
+
+## Quick Start Templates
+
+### Action Plugin
+
+```python
+"""
+title: My Action
+author: Your Name
+version: 1.0.0
+"""
+
+class Action:
+    async def action(self, body: dict, __event_emitter__=None):
+        await __event_emitter__({"type": "notification", "data": {"content": "Hello!"}})
+        return body
+```
+
+### Filter Plugin
+
+```python
+"""
+title: My Filter
+author: Your Name
+version: 1.0.0
+"""
+
+class Filter:
+    async def inlet(self, body: dict, __metadata__: dict) -> dict:
+        # Process before LLM
+        return body
+    
+    async def outlet(self, body: dict, __metadata__: dict) -> dict:
+        # Process after LLM
+        return body
+```
+
+### Pipe Plugin
+
+```python
+"""
+title: My Pipe
+author: Your Name
+version: 1.0.0
+"""
+
+class Pipe:
+    def pipes(self):
+        return [{"id": "my-model", "name": "My Model"}]
+    
+    def pipe(self, body: dict):
+        return "Response from custom pipe"
+```
+
+---
+
+## Core Concepts
+
+### Valves Configuration
+
+Valves allow users to configure plugins through the UI:
+
+```python
+from pydantic import BaseModel, Field
+
+class Action:
+    class Valves(BaseModel):
+        api_key: str = Field(default="", description="API Key")
+        enabled: bool = Field(default=True, description="Enable plugin")
+    
+    def __init__(self):
+        self.valves = self.Valves()
+```
+
+### Event Emitter
+
+Send notifications and status updates:
+
+```python
+# Notification
+await __event_emitter__({
+    "type": "notification",
+    "data": {"type": "success", "content": "Done!"}
+})
+
+# Status update
+await __event_emitter__({
+    "type": "status",
+    "data": {"description": "Processing...", "done": False}
+})
+```
+
+### User Context
+
+Access user information:
+
+```python
+user_name = __user__.get("name", "User")
+user_id = __user__.get("id")
+user_language = __user__.get("language", "en-US")
+```
+
+---
+
+## Best Practices
+
+1. **Async Operations**: Always use `async/await` for I/O operations
+2. **Error Handling**: Catch exceptions and notify users
+3. **Status Updates**: Provide feedback during long operations
+4. **Configuration**: Use Valves for customizable options
+5. **Documentation**: Include clear docstrings and README files
+
+---
+
+## Resources
+
+- [Full Development Guide](plugin-guide.md)
+- [Plugin Examples](https://github.com/Fu-Jie/awesome-openwebui/tree/main/plugins)
+- [OpenWebUI Documentation](https://docs.openwebui.com/)