Systemd has an excellent reference documentation in its manpages, but it does a lot of things, and a reference documentation isn't the best starting point for getting introduced to them.
I would like to see a bit more documentation of the kind that
sits between a
systemctl start|stop|status and the reference manpages.
Things like simple HOWTO posts on how to get a simple job done, or high-level
explanations of how some specific feature works.
I put some of what I know and used (or wrote) into systemd/documentation, I'll try to add to it when I find more, and I encourage you to do the same.
If you remember an article that has been useful to you and it is missing from the page, link it.
If you found a systemd feature useful to get a task done, write a little HOWTO to the wiki or your blog, and link it.
If you don't use systemd, go ahead and start a similar index for the init system that you use, and link it, too.
I'd love it if each init system we have in Debian had an excellent documentation index in the wiki!