Add comprehensive event types table to custom visualizer documentation

Add a table overview of all event types handled by the default visualizer
before diving into detailed descriptions. This provides a quick reference
for developers to understand the complete event system.

Co-authored-by: openhands <[email protected]>

Author: openhands-agent

sdk/guides/convo-custom-visualizer.mdx

@@ -297,7 +297,19 @@ class MinimalProgressVisualizer(ConversationVisualizer):
 
 ## Key Event Types
 
-Understanding the event system is crucial for effective custom visualizers:
+Understanding the event system is crucial for effective custom visualizers. Here's a comprehensive overview of all event types handled by the default visualizer:
+
+| Event Type | Description | Key Properties | When It Occurs |
+|------------|-------------|----------------|----------------|
+| `SystemPromptEvent` | System-level prompts and instructions | `content`, `source` | Agent initialization, system messages |
+| `ActionEvent` | Agent actions and tool calls | `action`, `tool_name`, `llm_response_id` | When agent decides to take an action |
+| `ObservationEvent` | Results from executed actions | `observation`, `source` | After action execution completes |
+| `MessageEvent` | LLM messages (user/assistant) | `llm_message` (role, content) | User input, agent responses |
+| `AgentErrorEvent` | Error conditions and failures | `error`, `source` | When agent encounters errors |
+| `PauseEvent` | User-initiated pauses | `source` | When user pauses conversation |
+| `Condensation` | Memory condensation events | `content`, `source` | During conversation memory management |
+
+### Detailed Event Descriptions
 
 ### ActionEvent
 Fired when the agent decides to use a tool or take an action.