Rebuild the docs around tested examples; shrink README.v2.md to a pitch (#2978)

docs_src/context/tutorial003.py

@@ -0,0 +1,17 @@
+from mcp.server import MCPServer
+from mcp.server.mcpserver import Context
+
+mcp = MCPServer("Bookshop")
+
+
+def recommend_book(genre: str) -> str:
+    """Recommend a book in the given genre."""
+    return f"In {genre}, try 'Dune'."
+
+
+@mcp.tool()
+async def enable_recommendations(ctx: Context) -> str:
+    """Switch on the recommendation tool."""
+    mcp.add_tool(recommend_book)
+    await ctx.session.send_tool_list_changed()
+    return "Recommendations are now available."