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 propertiesDocumentation:
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