docs(common): improving documentation

- update esp_modem to use esp_docs
- migrated docs from github pages to docs.espressif.com

docs/esp_modem/en/internal_docs.rst

@@ -0,0 +1,117 @@
+DCE Internal implementation
+===========================
+
+This chapter provides a detailed description of the classes and building blocks of the esp-modem component and their responsibilities.
+
+The esp-modem actually implements the DCE class, which in turn aggregates these thee units:
+
+- :ref:`DTE<dte_impl>` to communicate with the device on a specific Terminal interface such as UART.
+- :ref:`Netif<netif_impl>` to provide the network connectivity
+- :ref:`Module<module_impl>` to define the specific command library
+
+Developers would typically have to
+
+* Add support for a new module
+* Implement a generic (common for all modules) AT command
+
+This is explained in the :ref:`Module<module_impl>` section, as :ref:`Adding new module or command<module_addition>`
+
+------------
+
+.. doxygengroup:: ESP_MODEM_DCE
+   :members:
+
+.. _dte_impl:
+
+DTE abstraction
+---------------
+
+DTE is a basic unit to talk to the module using a Terminal interface. It also implements and uses the CMUX to multiplex
+terminals. Besides the DTE documentation, this section also refers to the
+
+- :ref:`Terminal interface<term_impl>`
+- :ref:`CMUX implementation<cmux_impl>`
+
+
+------------
+
+.. doxygengroup:: ESP_MODEM_DTE
+   :members:
+
+.. _term_impl:
+
+Terminal interface
+^^^^^^^^^^^^^^^^^^
+
+.. doxygengroup:: ESP_MODEM_TERMINAL
+   :members:
+
+.. _cmux_impl:
+
+CMUX implementation
+^^^^^^^^^^^^^^^^^^^
+
+.. doxygengroup:: ESP_MODEM_CMUX
+   :members:
+
+.. _netif_impl:
+
+Netif
+-----
+
+.. doxygengroup:: ESP_MODEM_NETIF
+   :members:
+
+.. _module_impl:
+
+Module abstraction
+------------------
+
+.. doxygengroup:: ESP_MODEM_MODULE
+   :members:
+
+.. _module_addition:
+
+Adding new devices
+^^^^^^^^^^^^^^^^^^
+
+To support a new module, developers would have to implement a new class derived from :cpp:class:`esp_modem::GenericModule` the same way
+as it is described in the  :ref:`Advanced user manual<create_custom_module>`. The only difference is that the new class (and factory extension)
+would be available in the esp_modem code base.
+
+Implement a new generic command
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding a generic command, i.e. the command that is shared for all modules and is included in the :cpp:class:`esp_modem::GenericModule`,
+has to be declared first in the ``include/generate/esp_modem_command_declare.inc`` file, which is the single source
+of supported command definitions, that is used in:
+
+* public C API
+* public CPP API
+* generated documentation
+* implementation of the command
+
+Therefore, a care must be taken, to correctly specify all parameters and types, especially:
+
+* Keep number of parameters low (<= 6, used in preprocessor's forwarding to the command library)
+* Use macros to specify parameter types (as they are used both from C and C++ with different underlying types)
+* Parameter names are used only for clarity and documentation, they get expanded to numbered arguments.
+
+Please use the following pattern: ``INT_IN(p1, baud)``, meaning that the parameter is an input integer,
+human readable argument name is ``baud``, it's the first argument, so expands to ``p1`` (second argument would be ``p2``, etc)
+
+Command library
+^^^^^^^^^^^^^^^
+
+This is a namespace holding a library of typical AT commands used by supported devices.
+Please refer to the :ref:`c_api` for the list of supported commands.
+
+.. doxygengroup:: ESP_MODEM_DCE_COMMAND
+   :members:
+
+
+Modem types
+-----------
+
+.. doxygengroup:: ESP_MODEM_TYPES
+   :members: