Fix NOTE admonitions to include code snippets

The single-line NOTE: shorthand only captures one line of text,
leaving the code block rendered outside the admonition. Use the
block form [NOTE]/==== to wrap the code snippet inside.

Author: sgerbino

doc/modules/ROOT/pages/3.tutorials/3a.echo-server.adoc

@@ -13,7 +13,9 @@ This tutorial builds a production-quality echo server using the `tcp_server`
 framework. We'll explore worker pools, connection lifecycle, and the launcher
 pattern.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/tcp_server.hpp>
@@ -23,6 +25,7 @@ NOTE: Code snippets assume:
 namespace corosio = boost::corosio;
 namespace capy = boost::capy;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/3.tutorials/3b.http-client.adoc

@@ -13,7 +13,9 @@ This tutorial builds a simple HTTP client that connects to a server, sends
 a GET request, and reads the response. You'll learn socket connection,
 composed I/O operations, and the exception-based error handling pattern.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio.hpp>
@@ -26,6 +28,7 @@ NOTE: Code snippets assume:
 namespace corosio = boost::corosio;
 namespace capy = boost::capy;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/3.tutorials/3c.dns-lookup.adoc

@@ -13,7 +13,9 @@ This tutorial builds a command-line DNS lookup tool similar to `nslookup`.
 You'll learn to use the asynchronous resolver to convert hostnames to IP
 addresses.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio.hpp>
@@ -23,6 +25,7 @@ NOTE: Code snippets assume:
 namespace corosio = boost::corosio;
 namespace capy = boost::capy;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/3.tutorials/3d.tls-context.adoc

@@ -13,13 +13,16 @@ This tutorial covers how to configure TLS contexts for secure connections.
 A `tls_context` stores certificates, keys, and settings that define how
 TLS connections are established and verified.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/tls_context.hpp>
 
 namespace tls = boost::corosio::tls;
 ----
+====
 
 == Introduction
 

doc/modules/ROOT/pages/4.guide/4c.io-context.adoc

@@ -13,12 +13,15 @@ The `io_context` class is the heart of Corosio. It's an event loop that
 processes asynchronous I/O operations, manages timers, and coordinates
 coroutine execution.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/io_context.hpp>
 namespace corosio = boost::corosio;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/4.guide/4d.sockets.adoc

@@ -13,7 +13,9 @@ The `tcp_socket` class provides asynchronous TCP networking. It supports
 connecting to servers, reading and writing data, and graceful connection
 management.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/tcp_socket.hpp>
@@ -23,6 +25,7 @@ NOTE: Code snippets assume:
 namespace corosio = boost::corosio;
 namespace capy = boost::capy;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/4.guide/4e.tcp-acceptor.adoc

@@ -12,7 +12,9 @@
 The `tcp_acceptor` class listens for incoming TCP connections and accepts them
 into socket objects. It's the foundation for building TCP servers.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/tcp_acceptor.hpp>
@@ -21,6 +23,7 @@ NOTE: Code snippets assume:
 
 namespace corosio = boost::corosio;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/4.guide/4f.endpoints.adoc

@@ -13,7 +13,9 @@ The `endpoint` class represents a network endpoint: an IP address (IPv4 or
 IPv6) combined with a port number. Endpoints are used for connecting sockets
 and binding acceptors.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/endpoint.hpp>
@@ -22,6 +24,7 @@ NOTE: Code snippets assume:
 
 namespace corosio = boost::corosio;
 ----
+====
 
 == Overview
 

doc/modules/ROOT/pages/4.guide/4g.composed-operations.adoc

@@ -12,7 +12,9 @@
 Corosio provides composed operations that build on the primitive `read_some()`
 and `write_some()` functions to provide higher-level guarantees.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/read.hpp>
@@ -22,6 +24,7 @@ NOTE: Code snippets assume:
 namespace corosio = boost::corosio;
 namespace capy = boost::capy;
 ----
+====
 
 == The Problem with Primitives
 

doc/modules/ROOT/pages/4.guide/4h.timers.adoc

@@ -14,13 +14,16 @@ The `timer` class provides asynchronous delays and timeouts. It integrates
 with the I/O context to schedule operations at specific times or after
 durations.
 
-NOTE: Code snippets assume:
+[NOTE]
+====
+Code snippets assume:
 [source,cpp]
 ----
 #include <boost/corosio/timer.hpp>
 namespace corosio = boost::corosio;
 using namespace std::chrono_literals;
 ----
+====
 
 == Overview