Here you can download binary distributions of the game for Linux or Windows compiled for x86_64, or download the source code and compile it yourself.
The game is released under the GNU AGPLv3.
Alternatively get the repository at https://codeberg.org/rustydev/spaceships.
Here you can download offline copies of the documentation. You can get the documentation for the latest version online here.
I haven't yet bothered compiling this for macOS. If you're using macOS, feel free to compile it yourself and see if it works.