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

docs_src/client_callbacks/tutorial004.py

@@ -0,0 +1,19 @@
+from mcp_types import CreateMessageRequestParams, CreateMessageResult, ListRootsResult, Root, TextContent
+from pydantic import FileUrl
+
+from mcp.client import ClientRequestContext
+
+
+async def handle_sampling(
+    context: ClientRequestContext,
+    params: CreateMessageRequestParams,
+) -> CreateMessageResult:
+    return CreateMessageResult(
+        role="assistant",
+        content=TextContent(type="text", text="The answer is 42."),
+        model="my-llm",
+    )
+
+
+async def handle_list_roots(context: ClientRequestContext) -> ListRootsResult:
+    return ListRootsResult(roots=[Root(uri=FileUrl("file:///home/ada/notebooks"), name="notebooks")])