It can be in git even if you’re not doing ‘config as code’ or ‘infrastructure as code’ yet/ever.
Even just a text file with notes in markdown is better than nothing. Can usually be rendered, tracked, versionned.
You can also add some relevant files as needed too.
Like, even if your stuff isn’t fully automated CI/CD magic, a copy of that one important file you just modified can be added as necessary.
Ultimately, do whatever you think you’ll be able to keep up with.
The best documentation system is useless if you keep putting it off because it’s too much work.