Added ADC API doc + simple example (#6301)

* Added ADC API doc + simple example

* Added attenuation input voltage range + conf.py added tabs extension

* Update requirements.txt

* Update adc.rst

libraries/ESP32/examples/AnalogRead/AnalogRead.ino

@@ -0,0 +1,19 @@
+void setup() {
+  // initialize serial communication at 115200 bits per second:
+  Serial.begin(115200);
+  
+  //set the resolution to 12 bits (0-4096)
+  analogReadResolution(12);
+}
+
+void loop() {
+  // read the analog / millivolts value for pin 2:
+  int analogValue = analogRead(2);
+  int analogVolts = analogReadMilliVolts(2);
+  
+  // print out the values you read:
+  Serial.printf("ADC analog value = %d\n",analogValue);
+  Serial.printf("ADC millivolts value = %d\n",analogVolts);
+  
+  delay(100);  // delay in between reads for clear read from serial
+}