[Docs] DAC API documentation

docs/source/api/dac.rst

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+###
+DAC
+###
+
+About
+-----
+
+DAC (digital to analog converter) is a very common peripheral used to convert a digital signal to an
+analog form. 
+
+ESP32 and ESP32-S2 have two 8-bit DAC channels. The DAC driver allows these channels to be set to arbitrary voltages.
+
+DACs can be used for generating a specific (and dynamic) reference voltage for external sensors,
+controlling transistors, etc.
+
+========= ========= =========
+ESP32 SoC DAC_1 pin DAC_2 pin
+========= ========= =========
+ESP32     GPIO 25   GPIO 26
+ESP32-S2  GPIO 17   GPIO 18
+========= ========= =========
+
+Arduino-ESP32 DAC API
+---------------------
+
+dacWrite
+********
+
+This function is used to set the DAC value for a given pin/DAC channel.
+
+.. code-block:: arduino
+
+    void dacWrite(uint8_t pin, uint8_t value);
+
+* ``pin`` GPIO pin.
+* ``value`` to be set. Range is 0 - 255 (equals 0V - 3.3V).
+
+dacDisable
+**********
+
+This function is used to disable DAC output on a given pin/DAC channel.
+
+.. code-block:: arduino
+
+    void dacDisable(uint8_t pin);
+
+* ``pin`` GPIO pin.