Ansible and Nix. Code is the document.

This is what I like about git ops and infra/config as Code personally.

Ideally everything is an a tofu/ansible/helm chart and git lab pipeline/Fleet job. I add comments for anything that I had to learn to make work to those files. Follow good commit hygenine (most of the time). And bam I can almost a year later half asleep stumble back into a thing I did.

Whenever I set something up I usually make a markdown file listing the commands and steps to take. I do this as I am setting things up and familiarizing myself, so once I'm done, I have a start to finish guide.

Raw text/markdown files will be readable until the end of time.

I agree with the advice that says “Document your setup such that you could recreate it from your notes from scratch” but I’d take it another step further — consider that someone may have to do some work on your system when you are unable or unavailable. The kind of thing you’d keep with your will, or power of attorney. Just a suggestion.

I believe it is traditional to do so written in blood in the style of an apocalypse log, dealer's choice for who's blood. Make sure it's disjointed and nearly incomprehensible, but that everything is there.

Bonus points if you print the config files and write your documentation on them after stapling them to the walls

I have two systems that sort of work together.

The first system involves a bunch of text files for each task. OS installation, basic post OS installation tasks and a file for each program I add (like UFW, apparmor, ddclient, docker and so on). They basically look like scripts with comments. If I want to I can just copy/paste everything into a terminal and reach a a specific state that I want to be at.

The second system is a sort of "skeleton" file tree that only contains all the files that I have added or modified.

.
├── etc
│   ├── crontabs
│   │   └── root
│   ├── ddclient
│   │   └── ddclient.conf
│   ├── doas.d
│   │   └── doas.conf
│   ├── fail2ban
│   │   ├── filter.d
│   │   │   └── alpine-sshd-key.conf
│   │   └── jail.d
│   │       └── alpine-ssh.conf
│   ├── modprobe.d
│   │   ├── backlist-extra.conf
│   │   └── disable-filesystems.conf
│   ├── network
│   │   └── interfaces
│   ├── periodic
│   │   └── 1min
│   │       └── dynamic-motd
│   ├── profile.d
│   │   └── profile.sh
│   ├── ssh
│   │   └── sshd_config
│   ├── wpa_supplicant
│   │   └── wpa_supplicant.conf
│   ├── fstab
│   ├── nanorc
│   ├── profile
│   └── sysctl.conf
├── home
│   └── pi-user
│       ├── .config
│       │   └── ash
│       │       ├── ashrc
│       │       └── profile
│       ├── .ssh
│       │   └── authorized_keys
│       ├── .sync
│       │   ├── file-system-backup
│       │   │   ├── .sync-server-fs_01_root
│       │   │   └── .sync-server-fs_02_boot
│       │   └── .sync-caddy_certs_backup
│       ├── .nanorc
│       └── .tmux.conf
├── root
│   ├── .config
│   │   └── mc
│   │       └── ini
│   ├── .local
│   │   └── share
│   │       └── mc
│   │           └── history -> /dev/null
│   ├── .ssh
│   │   └── authorized_keys
│   ├── scripts
│   │   ├── automated-backup
│   │   └── maintenance
│   ├── .ash_history -> /dev/null
│   └── .nanorc
├── srv
│   ├── caddy
│   │   ├── Caddyfile
│   │   ├── Dockerfile
│   │   └── docker-compose.yml
│   └── kiwix
│       └── docker-compose.yml
└── usr
    └── sbin
        ├── containers-down
        ├── containers-up
        ├── emountman
        ├── fs-backup-quick
        └── rtransfer

This is useful to me because I can keep track of every change I make. I even have it set up so I can use rsync to quickly chuck all the files into place after a fresh install or after adding/modifying files.

I also created and maintain a "quick install" guide so I can install a fresh OS, rsync all the modified files from my skeleton file tree into place, then run through all the commands in my quick install guide to get myself back to the same state in a minimal amount of time.

I created something similar to this. It got a lot of love during interviews later down the line. https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Fi.redd.it%2Fvmd34mabi4r91.jpg&f=1&ipt=2dde77fd04d48156bc514ad4b1f090c8473f4e666ead0e16906eeed55a79aca6

I'm not real clear what exactly you need to document.
Infrastructure documentation starts with an IPAM.
A good IPAM can help you document all kinds of stuff.

I use NetBox.
https://github.com/netbox-community/netbox?tab=readme-ov-file#getting-started

I'm running it as a Docker container on a Linux VM.
I just looked at their latest screenshots, and it appears they've done quite a bit with it since I stood up my copy.
It does even more now. I'll have to upgrade.