5V (analog) to 3.3V (analog) for current measuring
What you want is a voltage divider, R7 and R8 in your link, where the the ESP8266 is the opamp.

To calculate the values of the resistors you need to know a couple of things:


Max Current going through the ACS723
Output voltage per A from the ACS723 mV/A
Reference Voltage of the ADC


With these elements you will know exactly if you need the resistors.
Looking at the datasheet, it does look that under normal operation, the maximum output range is 0-4V with the 0A voltage depending on the model you select, but as an example lets assume that your maximum current is 7A, and therefore you select the 10A part, which has 400mV/A output. 

This means that at 7A, the output from the ACS723 will be 2.8V

Now, for what I've researched, the ESP8266 uses VCC as a reference voltage, and therefore it is 3V3 in your case, which means that you don't need a voltage divider, however, I would put a single current limiting resistor and a 3V3 Zener to clamp the spikes if the current was to exceed 8.25V.
Theoretically you can use a divider, but then you would limit the resolution of the ADC.

The ESP8266 has a 10bit resolution ADC, where 0V=0 and 3.3V=1024, which translates that you will be wasting the ADC range between 2.8V and 3.3V.

The older versions of the ESP8266 used to have a 1V reference voltage, which meant that you could use the ADC at its full potential by adjusting the resistor partition.

Now lets assume that you will need the full range 0-4V, in that case you will need a voltage divider to limit the voltage on the ADC pin.
To calculate this do this

Vout= (Vin x R8)/(R7 + R8)

With this we can get that when R7 is 820 Ohm and R8 is 3.9k, your maximum voltage on the ADC at 4V will be 3.305V.

You know your application, so you will know your range, however, I recommend that you look up how ADCs work, so that you get exactly the results you expect.