I don’t remember all the exact steps, but I have roughly followed the instructions on https://helpcenter.onlyoffice.com/server/linux/document/compile-source-code.aspx.
Especially the first step (compiling 3rd party apps) was a bit challenging because it makes use of precompiled executables and library files by default, which I had to compile or download manually for ARM. This includes ICU, CEF and V8 (the first two I compiled on my board, V8 I cross-compiled on my laptop).
For the rest, I was not able to use GCC 4.9, because I got an internal compiler error when compiling core, so I switched to GCC 6.
This is what I remember, if you run into other problems please feel free to ask. Would be interesting to see if you get a fully working DocumentServer or whether you can reproduce the problems I mention in the previously referenced threads.