doc: Combine all PPP pages into one "Dial-up modem" document

Describe the PPP behavior in one page, instead of spreading the
information between two separate pages.

Remove separated pages for Linux and Zephyr.
Move trace-collecting through CMUX to a logging page.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

doc/app/PPP_linux.rst

@@ -67,8 +67,7 @@ See the subpages for how to use the application, how to extend it, and informati
    sm_build_and_run
    sm_logging
    sm_configuration
-   nRF91_as_Zephyr_modem
-   PPP_linux
+   sm_cellular_modem
    sm_data_mode
    sm_extending
    sm_testing

doc/app/README.rst

@@ -16,8 +16,7 @@ The maximum length of the information field in UIH frames is configurable using
 
 .. note::
 
-   To use the nRF91 Series SiP as a standalone modem in Zephyr, see :ref:`sm_as_zephyr_modem`.
-   To use the nRF91 Series SiP as a modem for a Linux device, see :ref:`sm_as_linux_modem`.
+   To use the nRF91 Series SiP as a cellular dial-up PPP modem, see :ref:`sm_cellular_modem`.
 
 CMUX is enabled in |SM| by compiling it with the appropriate configuration files, depending on your use case.
 See the :ref:`sm_config_files` section for more information.

doc/app/at_cmux.rst

@@ -9,10 +9,6 @@ PPP AT commands
 
 This page describes AT commands related to the Point-to-Point Protocol (PPP).
 
-.. note::
-
-   To use the nRF91 Series SiP as a standalone modem in Zephyr, see the :ref:`sm_as_zephyr_modem` documentation.
-
 PPP is enabled in |SM| by compiling it with the appropriate configuration files, depending on your use case (with or without CMUX).
 See the :ref:`sm_config_files` section for more information.
 
@@ -228,7 +224,7 @@ Testing on Linux
 
 You can test |SM|'s PPP on Linux by using the ``pppd`` command.
 This section describes a configuration without CMUX.
-If you are using CMUX, see :ref:`sm_as_linux_modem` for more information on setting it up.
+If you are using CMUX, see :ref:`sm_cellular_modem` for more information on setting it up.
 
 For the process described here, |SM|'s UARTs must be connected to the Linux host.