docs: update the table of contents and actor/stack images (#1080)

Replaced the images and restructured the TOC.

Closes #1070

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Holt Skinner <[email protected]>

Author: 3 people

docs/assets/a2a-actors.png

@@ -1,4 +1,4 @@
-# A2A Protocol Overview
+# What is A2A?
 
 The A2A protocol is an open standard that enables seamless communication and
 collaboration between AI agents. It provides a common language for agents built

docs/assets/agentic-stack.png

@@ -1,5 +1,5 @@
 # Project information
-site_name: A2A Protocol Documentation
+site_name: A2A Protocol
 site_url: https://a2a-protocol.org/
 site_description: >-
   The Agent2Agent protocol is an open standard that allows different AI agents to securely communicate, collaborate, and solve complex problems together.
@@ -15,22 +15,16 @@ extra:
 # Navigation
 nav:
   - Home: index.md
-  - Topics:
-    - A2A Overview: topics/what-is-a2a.md
+  - Documentation:
+    - What is A2A?: topics/what-is-a2a.md
     - Core Concepts: topics/key-concepts.md
-    - A2A and MCP: topics/a2a-and-mcp.md
     - Agent Discovery: topics/agent-discovery.md
     - Enterprise Features: topics/enterprise-ready.md
-    - Streaming & Asynchronous Operations: topics/streaming-and-async.md
-    - Extensions: topics/extensions.md
     - Life of a Task: topics/life-of-a-task.md
-  - Specification: specification.md
-  - Community: community.md
-  - Partners: partners.md
-  - SDK Reference:
-    - sdk/index.md
-    - Python: sdk/python/api/index.html
-  - Tutorials:
+    - Extensions: topics/extensions.md
+    - Streaming & Asynchronous Operations: topics/streaming-and-async.md
+    - A2A and MCP: topics/a2a-and-mcp.md
+  - Tutorials and Samples:
     - tutorials/index.md
     - Quickstart (Python):
       - Introduction: tutorials/python/1-introduction.md
@@ -41,6 +35,12 @@ nav:
       - Interact with Server: tutorials/python/6-interact-with-server.md
       - Streaming & Multiturn: tutorials/python/7-streaming-and-multiturn.md
       - Next Steps: tutorials/python/8-next-steps.md
+  - Specification: specification.md
+  - SDK Reference:
+    - sdk/index.md
+    - Python: sdk/python/api/index.html
+  - Community: community.md
+  - Partners: partners.md
   - Roadmap: roadmap.md
 
 # Repository