I run a k3s cluster for selfhosted apps and keep all the configuration and docs in a git repo. That way I have history of changes and can rollback if needed. In that repo I have a docs folder with markdown documents about common operations and runbooks.
There are other ways to do this, but I like keeping docs next to the code and config so I can update them all at the same time. Deployed several wikis in the past but always forget to update them when I change things.
https://forgejo.org selfhosted has been good for me, FOSS fork of Gitea.