Yes, that document is outdated – I didn’t get a chance to look at it while I was working on the documentation. Sorry for leaving the old information there.
On a more basic web server configuration that is automatic. In Firefox type Ctrl/I (or Tools/Page Info) and look at the “Modified:” field, which in turn is from the “Last-Modified:” HTTP response header.
I expect that you wanted the date to appear within the displayed content of the page itself.
Maybe Purism runs a Content Management System that manages to make that not work however. But that should mean that you can get the date from the edit history of the underlying source - if you can make head or tail of it.
The user and development documentation are generated using Sphinx, which is popular with various projects these days but suboptimal for a lot of documentation needs.
Anyway, the problem is that it may well be possible to use Plasma Mobile with PostmarketOS on the Librem 5 but I’m not sure what would need to be done to make it work on top of PureOS on a stock Librem 5. Does anyone have experience with this?