I set up a folder for notes that shares across my network using SyncThing and is backed up with a FreeNAS box.

That folder is just a collection of markdown files for each program / system and when I save on one device it updates the documentation on them all.

I use Atom to view and edit them on my Linux machines and a markdown editor app on my phone. This allows me to search across the notes too.

I've had this fairly simple, free, open source setup for years with no problems.