The datasheet shows that the unit has an IF output. The output frequency of the IF will be proportional to speed of the object being measured. Unfortunately the datasheet doesnt give any info on what that would be.
You would have to connect the IF out to a frequency counter probably via a preamp and do some tests to get an idea of the range of the IF out frequency compared to the monitored object's speed
The formula listed is f = fo* [( C + Vr)/(C + Vs)], where C is wave velocity in medium, if free space @ 10.5 GHz would be close to 2.998E10 cm/sec, Vr is the velocity of receiver relative to medium, Vs is the transmitter velocity relative to medium.
The unit is a 'homodyne' receiver, basically just mixes the received signal with transmitted signal to yield the difference beat tone between the two.
So its beat frequency output will be 2 times 10.5 GHz times the absolute velocity relative to a fixed object / speed of light. The amplitude will be a function of the strength of the reflected signal. Metal objects will be stronger then a hand.
For example if you throw a ball of aluminum foil at the unit at 20 km per hour (555.56 cm/sec) you would get 2 * 10.5 GHz * (555.56 cm/sec) / (2.998 x 10^10 cm/sec) = 389 Hz tone output. Hook the output up to input of audio amp and throw alum foil balls at unit to hear the music it makes.
you need to amplify the IF out. It will be an analog signal, below 1 MHz. Use a good op amp. Then you need to figure out its frequency. If there are multiple targets, you will want some sort of DSP processor to measure the frequencies of the various tones in the IF, and figure out which one is the target's that you desire.
If you do not care about clutter, u could just assume the highest frequency tone is your targets, and do some analog or digital processing to get its frequency (like a frequency to voltage detector, lumped element frequency discriminator, frequency counter, etc)