docs(common): improving documentation

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

docs/esp_modem/en/api_docs.rst

@@ -0,0 +1,57 @@
+.. _c_api:
+
+C API Documentation
+===================
+
+
+The C API is very simple and consist of these two basic parts:
+
+- :ref:`lifecycle_api`
+- :ref:`modem_commands`
+
+The Typical application workflow is to:
+
+- Create a DCE instance (using :cpp:func:`esp_modem_new`)
+- Call specific functions to issue AT commands (:ref:`modem_commands`)
+- Switch to the data mode (using :cpp:func:`esp_modem_set_mode`)
+- Perform desired network operations (using standard networking API, unrelated to ESP-MODEM)
+- Optionally switch back to command mode (again :cpp:func:`esp_modem_set_mode`)
+- Destroy the DCE handle (sing :cpp:func:`esp_modem_destroy`)
+
+Note the configuration structures for DTE and DCE, needed for creating the DCE instance, is documented in :ref:`api_config`
+
+.. _lifecycle_api:
+
+Lifecycle API
+-------------
+
+These functions are used to create, destroy and set modem working mode.
+
+- :cpp:func:`esp_modem_new`
+- :cpp:func:`esp_modem_destroy`
+- :cpp:func:`esp_modem_set_mode`
+
+.. doxygengroup:: ESP_MODEM_C_API
+
+
+.. _modem_commands:
+
+Modem commands
+--------------
+
+These functions are the actual commands to communicate with the modem using AT command interface.
+
+Note that the functions which implement AT commands returning textual values use plain ``char *``
+pointer as the return value. The API expects the output data to point to user allocated space of at least
+``ESP_MODEM_C_API_STR_MAX`` (64 by default) bytes, it also truncates the output data to this size.
+
+.. doxygenfile:: esp_modem_api_commands.h
+
+.. _api_config:
+
+Configuration structures
+------------------------
+
+
+.. doxygengroup:: ESP_MODEM_CONFIG
+   :members: