For storing the API docs offline @RosemaryOrchard suggested Offline Pages Pro. While this App works, it has not been updated for more than a year. As a result it does not support the full screen of the iPhone X 
(black bars instead)
Are there any more recent alternatives to this App? Which?
(I’d still prefer a Docset for Dash…)
I realize that you’re probably looking to store the docs in a separate app so you can make use of split view on the iPad when you’re offline. However, I wanted to mention that the entire documentation is also available offline in the app.
Yes, that’s what I would like. But having the website online is already nice. Thanks!
Also, if your publishing to website process could make a PDF that would be grand. (Yes I know that might be difficult.)
You can use Devonthink Pro or Devonthink Pro Office to create an offline archive of the API which is completely browsable offline within Devonthink (and thus also in the IOS Devonthink to Go app).
The post referred is 6 years old, but still working 
Probably not perfectly fomatted, but PDF Pen Pro on the Mac can capture sites to PDF. That could even be scheduled for periodic generation.
I’m petty sure judicious use of wget and pandoc could probably work wonders from the command line on a number of platforms and again get something resembling a workable reference PDF produced.
Similarly it would be trivial to grab a list of the page URLs for the docs and then loop over them in Shortcuts on iOS to grab the page content (full, a simplified article version, or a stripped down Markdown version for example), render each page to a PDF version and then combine those PDFs.
I use the Dash app for this, it would be great to have Scriptable in there too.
I do like how the documentation has been implemented in-app; it’s one of the best I’ve seen.
It will be nice to have a split screen in the app, left side for coding and right side for docs, similar to Swift Playgrounds