Problem with photodiode luminance measurement
the problem is that I don't know how to relate the current reading of
the photodiode with the intensity of light detected. Looking in the
datasheet, I don't find a function of these two quantities either

It's the graph of reverse diode current vs light intensity that you should look at: -

So, you get 80 μA/mW/cm². If you know the active area of your photodiode then you can get a value in μA/watt (the more conventional figure for photodiodes). Oh, that's quoted on page 1 as 7.5 mm² or 0.5625² cm.
That equals 0.5625 x 80 μA/mW = 45 μA/mW = 45 mA/watt.