[Docs] Added USB documentation (#6036)

Summary

Added USB documentation.

Closes: #5784

docs/source/api/usb_cdc.rst

@@ -0,0 +1,192 @@
+#######
+USB CDC
+#######
+
+About
+-----
+
+USB Communications Device Class API. This class is used to enable communication between the host and the device.
+
+This class is often used to enable serial communication and can be used to flash the firmware on the ESP32 without the external USB to Serial chip.
+
+APIs
+****
+
+onEvent
+^^^^^^^
+
+Event handling functions.
+
+.. code-block:: arduino
+
+    void onEvent(esp_event_handler_t callback);
+
+.. code-block:: arduino
+
+    void onEvent(arduino_usb_cdc_event_t event, esp_event_handler_t callback);
+
+Where ``event`` can be:
+
+* ARDUINO_USB_CDC_ANY_EVENT
+* ARDUINO_USB_CDC_CONNECTED_EVENT
+* ARDUINO_USB_CDC_DISCONNECTED_EVENT
+* ARDUINO_USB_CDC_LINE_STATE_EVENT
+* ARDUINO_USB_CDC_LINE_CODING_EVENT
+* ARDUINO_USB_CDC_RX_EVENT
+* ARDUINO_USB_CDC_TX_EVENT
+* ARDUINO_USB_CDC_MAX_EVENT
+
+setRxBufferSize
+^^^^^^^^^^^^^^^
+
+The ``setRxBufferSize`` function is used to set the size of the RX buffer.
+
+.. code-block:: arduino
+
+    size_t setRxBufferSize(size_t size);
+
+setTxTimeoutMs
+^^^^^^^^^^^^^^
+
+This function is used to define the time to reach the timeout for the TX.
+
+.. code-block:: arduino
+
+    void setTxTimeoutMs(uint32_t timeout);
+
+begin
+^^^^^
+
+This function is used to start the peripheral using the default CDC configuration.
+
+.. code-block:: arduino
+
+    void begin(unsigned long baud);
+
+Where:
+
+* ``baud`` is the baud rate.
+
+end
+^^^
+
+This function will finish the peripheral as CDC and release all the allocated resources. After calling ``end`` you need to use ``begin`` again in order to initialize the USB CDC driver again.
+
+.. code-block:: arduino
+
+    void end();
+
+available
+^^^^^^^^^
+
+This function will return if there are messages in the queue.
+
+.. code-block:: arduino
+    
+    int available(void);
+
+The return is the number of bytes available to read.
+
+availableForWrite
+^^^^^^^^^^^^^^^^^
+
+This function will return if the hardware is available to write data.
+
+.. code-block:: arduino
+
+    int availableForWrite(void);
+
+peek
+^^^^
+
+This function is used to ``peek`` messages from the queue.
+
+.. code-block:: arduino
+
+    int peek(void);
+
+read
+^^^^
+
+This function is used to read the bytes available.
+
+.. code-block:: arduino
+
+    size_t read(uint8_t *buffer, size_t size);
+
+Where:
+
+* ``buffer`` is the pointer to the buffer to be read.
+* ``size`` is the number of bytes to be read.
+
+write
+^^^^^
+
+This function is used to write the message.
+
+.. code-block:: arduino
+
+    size_t write(const uint8_t *buffer, size_t size);
+
+Where:
+
+* ``buffer`` is the pointer to the buffer to be written.
+* ``size`` is the number of bytes to be written.
+
+flush
+^^^^^
+
+This function is used to flush the data.
+
+.. code-block:: arduino
+
+    void flush(void);
+
+baudRate
+^^^^^^^^
+
+This function is used to get the ``baudRate``.
+
+.. code-block:: arduino
+
+    uint32_t baudRate();
+
+setDebugOutput
+^^^^^^^^^^^^^^
+
+This function will enable the debug output, usually from the `UART0`, to the USB CDC.
+
+.. code-block:: arduino
+
+    void setDebugOutput(bool);
+
+enableReboot
+^^^^^^^^^^^^
+
+This function enables the device to reboot by the DTR as RTS signals.
+
+.. code-block:: arduino
+
+    void enableReboot(bool enable);
+
+rebootEnabled
+^^^^^^^^^^^^^
+
+This function will return if the reboot is enabled.
+
+.. code-block:: arduino
+
+    bool rebootEnabled(void);
+
+Example Code
+------------
+
+Here is an example of how to use the USB CDC.
+
+USBSerial
+*********
+
+.. literalinclude:: ../../../libraries/USB/examples/USBSerial/USBSerial.ino
+    :language: arduino
+
+.. _USB.org: https://www.usb.org/developers