docs/esp8266: Add ESP8266 tutorial.

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+Analog to Digital Conversion
+============================
+
+The ESP8266 has a single pin (separate to the GPIO pins) which can be used to
+read analog voltages and convert them to a digital value.  You can construct
+such an ADC pin object using::
+
+    >>> import machine
+    >>> adc = machine.ADC(0)
+
+Then read its value with::
+
+    >>> adc.read()
+    58
+
+The values returned from the ``read()`` function are between 0 (for 0.0 volts)
+and 1024 (for 1.0 volts).  Please note that this input can only tolerate a
+maximum of 1.0 volts and you must use a voltage divider circuit to measure
+larger voltages.