A document is the actual piece of paper or the computer file that contains the information. It is countable and it is also a verb.
I.e.: The scientists documented the lab experiments to prove the herb's medicinal properties
Documentation is the act of writing that information down.
Often used to describe the gathering and compilation of information from studies or research. It is NON-countable and it is NOT a verb.
I.e.: This is just one report, where is the documentation for the whole project.
Source: HiNative, Stackexchange