Added simple docs sketch

esp_modem/docs/api_docs.rst

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+.. _c_api:
+
+API Guide for C interface
+=========================
+
+
+C API is very simple and consist of these two basic parts:
+
+- :ref:`lifecycle_api` 
+- :ref:`modem_commands`
+
+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`)
+
+
+.. _lifecycle_api:
+
+Lifecycle API
+-------------
+
+These functions are used to create, destroy and set modem working 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.
+
+
+.. doxygenfile:: c_api.h