docs(common): improving documentation

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

docs/esp_modem/en/cxx_api_docs.rst

@@ -0,0 +1,44 @@
+C++ API Documentation
+=====================
+
+Similar to the :ref:`c_api`, the basic application workflow consist of
+
+.. toctree::
+
+- :ref:`Construction of the DCE<cpp_init>`
+- :ref:`Switching modes<cpp_mode_switch>`
+- :ref:`Sending (AT) commands<cpp_modem_commands>`
+- :ref:`Destroying the DCE<cpp_destroy>`
+
+.. _cpp_init:
+
+Create DTE and DCE
+------------------
+
+.. doxygengroup:: ESP_MODEM_INIT_DTE
+
+.. doxygengroup:: ESP_MODEM_INIT_DCE
+
+
+.. _cpp_mode_switch:
+
+Mode switching commands
+-----------------------
+
+.. doxygenclass:: esp_modem::DCE_T
+   :members:
+
+.. _cpp_modem_commands:
+
+Modem commands
+--------------
+
+.. include:: cxx_api_links.rst
+
+.. _cpp_destroy:
+
+Destroy the DCE
+---------------
+
+The DCE object is created as ``std::unique_ptr`` by default and as such doesn't have to be explicitly destroyed.
+It simply gets destroyed and cleaned-up automatically if the object goes out of the block scope.