For offline documentation, I use these in order of preference:
• Info¹ documentation, which I read directly in Emacs. (If you have ever used the terminal-based standalone “info” program, please try to forget all about it. Use Emacs to read Info documentation, and preferably use a graphical Emacs instead of a terminal-based one; Info documentation occasionally has images.)
• Gnome Devhelp².
• Zeal³
• RFC archive⁴ dumps provided by the Debian “doc-rfc“ package⁵.
[1] https://github.com/freeCodeCamp/devdocs
[2] https://zealdocs.org/