docs: Add Step/Chain-of-Thought UI documentation

Add comprehensive documentation for existing Step/Chain-of-Thought functionality
to achieve parity with Chainlit's cl.Step for PraisonAI migration.

The Step feature was already fully implemented:
- Backend events: RunEventType.REASONING_* in provider.py
- Frontend UI: ThinkingSteps.tsx + ToolCallDisplay.tsx
- Python API: aiui.Step async context manager in message.py
- Nested steps: Parent/child hierarchy support
- Streaming: Real-time content updates

Added docs/features/steps.md with:
- Basic usage examples and nested step patterns
- Common step types (tool_call, reasoning, sub_agent, retrieval, custom)
- API reference and frontend visualization details
- Chainlit migration guide for cl.Step replacement

Closes #2

Co-authored-by: Mervin Praison <[email protected]>

docs/features/steps.md

@@ -0,0 +1,113 @@
+# Steps / Chain-of-Thought
+
+Visualize agent reasoning and tool calls as collapsible, nested steps in the chat interface.
+
+## Basic Usage
+
+```python
+import praisonaiui as aiui
+
+@aiui.reply
+async def agent_response(message):
+    # Tool call step
+    async with aiui.Step(name="🔧 Tool: web_search") as step:
+        await step.stream_token("Input: { query: 'rust async ecosystem 2026' }")
+        # ... perform actual tool call
+        await step.stream_token("Output: [ 10 search results ]")
+    
+    # Reasoning step
+    async with aiui.Step(name="🧠 Reasoning: Synthesizing results") as step:
+        await step.stream_token("I see three major async runtimes...")
+    
+    return "Rust's async ecosystem in 2026 is dominated by..."
+```
+
+## Nested Steps
+
+Steps can be nested to show hierarchical reasoning:
+
+```python
+async with aiui.Step(name="Multi-agent research") as parent:
+    await parent.stream_token("Starting research task...")
+    
+    # Sub-agent step
+    async with aiui.Step(name="🤖 Sub-agent: Technical Analysis", parent=parent) as sub:
+        await sub.stream_token("Analyzing technical details...")
+        
+        # Tool called by sub-agent
+        async with aiui.Step(name="🔧 Tool: code_search", parent=sub) as tool:
+            await tool.stream_token("Searching codebase...")
+```
+
+## Step Types
+
+Common step patterns:
+
+```python
+# Tool calls
+async with aiui.Step(name="🔧 Tool: database_query") as step:
+    await step.stream_token("Querying user database...")
+
+# Reasoning/thinking
+async with aiui.Step(name="🧠 Reasoning: Plan evaluation") as step:
+    await step.stream_token("Evaluating different approaches...")
+
+# Sub-agents
+async with aiui.Step(name="🤖 Sub-agent: Code reviewer") as step:
+    await step.stream_token("Reviewing code quality...")
+
+# Retrieval
+async with aiui.Step(name="📚 Retrieval: Knowledge search") as step:
+    await step.stream_token("Searching knowledge base...")
+
+# Custom
+async with aiui.Step(name="⚡ Custom: Data processing") as step:
+    await step.stream_token("Processing large dataset...")
+```
+
+## Frontend Visualization
+
+Steps appear in the chat as collapsible panels showing:
+
+- **Icons** indicating step type (🔧 🧠 🤖 📚 ⚡)
+- **Expandable content** with input/output details
+- **Nested hierarchy** for complex reasoning chains
+- **Real-time updates** as steps stream content
+
+## API Reference
+
+### `aiui.Step(name, parent=None, metadata={})`
+
+**Parameters:**
+- `name` (str): Display name for the step
+- `parent` (Step, optional): Parent step for nesting
+- `metadata` (dict, optional): Additional step metadata
+
+**Methods:**
+- `async stream_token(token)`: Stream content to the step
+- Context manager protocol: `async with step: ...`
+
+### Events Emitted
+
+Steps emit these `RunEventType` events:
+- `REASONING_STARTED`: Step begins
+- `REASONING_STEP`: Content streamed 
+- `REASONING_COMPLETED`: Step finishes
+
+## Chainlit Migration
+
+Direct replacement for `cl.Step`:
+
+```python
+# Chainlit
+async with cl.Step(name="Tool call", type="tool") as step:
+    step.input = {"query": "rust async"}
+    # ... tool execution
+    step.output = ["result1", "result2"]
+
+# PraisonAIUI equivalent
+async with aiui.Step(name="🔧 Tool call") as step:
+    await step.stream_token("Input: { query: 'rust async' }")
+    # ... tool execution  
+    await step.stream_token("Output: ['result1', 'result2']")
+```