Does it exist?
I also wouldn't mind on-device documentation like man pages.
I have searched this for a few days, here, on TJC, on jolla.com and sailfish.org and even wiki.merproject.org. There's a sailfishos wiki, but it's very developer-specific afaics, not for (power) users like me.
There is a lot of nice information out there, but is there a main place to look for it, or do I have to trawl these half dozen sites anytime I'm looking for a solution?

This is not a rant or complaint, I really need an answer to this question before I can move on.

What do you think as a User Documentation?
User manual?
There is a Tutorial app in SailfishOS and hints when you trying to make something for a first time.
You can find movies on jolla's youtube channel but some of them are outdated:
https://www.youtube.com/user/jollaofficial/videos

You can find some guides on jolla's zendesk, form example:
https://jolla.zendesk.com/hc/en-us/c...duct-Usability
https://jolla.com/guide/en/index.html

There is a Tutorial, if that counts. In fact you can't get rid of it.

manpages.

I don't buy the idea that packing manpages with the OS would somehow be a waste of space on a mobile device. I mean, the whole /usr/share/man hierarchy on my desktop is just 54M, ridiculous!

I should have specified: power user documentation.
I don't much like the term, but I'd consider myself a power user under Linux at least. Not a coder/developer, but not an untechnical user either.
Familiar with editing config files and using the command line.

Jolla's zendesk seems to add one more site to my collection of links.

I am totally unfamilar with SailfishOS's package manager - how can I list installed packages, or list the files of one installed package (on the command line of course)? There might be some documentation there somewhere.

Jolla uses pkcon by default. You can find a lot of manuals and examples for pkcon in internet.
If you want to use command line you should start to use manuals.

pkcon as well as zypper (separate/manual installation) is the thing to search for manuals on the net.
And underlying is rpm packaging, so a deeper understanding/knowledge of 'rpm' is even better. (did not yet get to it, apt I know of )

list all packages:
rpm -qa

list package belonging to a file
rpm -qf filename

list files of a package
rpm -ql package

and so on ...

^ ah, thanks!
https://linux.die.net/man/8/rpm

And this is what we would like to have on device!

COME ON @JOLLA
How hard is this???

One way to handle it would be to build all packages with manuals, and then strip the mans to another set of rpms, just like debuginfo of packages.
Then, the people who would like to have manpages could pull all the rpms in via metapackage.