doc: connectivity: networking: api: Add subsystem link notes to CoAP

The CoAP documentation has server/clients sample usages, added a
note about the available subsystems that can be used instead.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>

doc/connectivity/networking/api/coap.rst

@@ -28,8 +28,6 @@ See :ref:`lwm2m_interface` for more information.
 
 Supported RFCs:
 
-Supported RFCs:
-
 - `RFC7252: The Constrained Application Protocol (CoAP) <https://tools.ietf.org/html/rfc7252>`_
 - `RFC6690: Constrained RESTful Environments (CoRE) Link Format <https://tools.ietf.org/html/rfc6690>`_
 - `RFC7959: Block-Wise Transfers in the Constrained Application Protocol (CoAP) <https://tools.ietf.org/html/rfc7959>`_
@@ -43,6 +41,11 @@ Sample Usage
 CoAP Server
 ===========
 
+.. note::
+
+   A :ref:`coap_server_interface` subsystem is available, the following is for creating a custom
+   server implementation.
+
 To create a CoAP server, resources for the server need to be defined.
 The ``.well-known/core`` resource should be added before all other
 resources that should be included in the responses of the ``.well-known/core``
@@ -97,6 +100,11 @@ with resource path like '/some_resource/+/#'.
 CoAP Client
 ===========
 
+.. note::
+
+   A :ref:`coap_client_interface` subsystem is available, the following is for creating a custom
+   client implementation.
+
 If the CoAP client knows about resources in the CoAP server, the client can start
 prepare CoAP requests and wait for responses. If the client doesn't know
 about resources in the CoAP server, it can request resources through