https://devdocs.io/offline can also download 600 documentation sets into the browser. I don't know how reliable it is over several months "Note: your browser may delete DevDocs's offline data if your computer is running low on disk space and you haven't used the app in a while."
If you're using firefox, create a separate profile and add devdocs documentation to it. That keeps your docs isolated from general browsing and so you reduce the risk of the browser deleting it to free up space.
Thank you for giving me the solution to this problem!
I always wanted devdocs to work (long train journeys especially would suddenly be more fun) but after this happened a couple of times and I couldn't figure it out or stop it I gave up.
Shame they don't offer something like an electron app with guaranteed persisted storage. Or even just a .zip containing everything, to be served from a local web server.
Maybe install it into a separate browser profile & make a backup of it just in case
