How to make a software documentation "officially" citable? We are currently finishing the implementation of a scientific software library. It's the first project of this type for me and I'm curious, how to make the software citable in a similar way to Molpro or R?

I was thinking about writing a short paper introducing the library, which could be subsequently cited. But I'm not sure, how to deal with the different versions? I don't think it's possible to write just some "changelog" paper once a year to summarize new released version.
There are about as many approaches to citing software (and making software citable) as there are software packages.

One way is to create a DOI for your software via Zenodo. This DOI can then be updated for each version of the software. Another way is to write a paper about the design and features of your software and see that it gets published somewhere. There are numerous journals for this -- for example, SoftwareX or, for mathematical software, the ACM Transactions on Mathematical Software. (Disclaimer: I'm a co-Editor-in-Chief of the latter.)

There are numerous other projects that have guidelines of how to best do this. You may want to take a look at the Force11 project, for example, as well as the outcomes of the WSSSPE series of workshops.