https://linuxjourney.com/ is a good place to learn about linux. For documentation, manpages for sure.

Unfortunately, Linux manuals are pretty scattered around. I’ll try to find something for you:

• If you want to learn the very basics of Linux command line, here’s what I found on the web: http://www.ee.surrey.ac.uk/Teaching/Unix/ ;
• For on how to use GNU user-land tools (Bash, cat, less, grep, sed, etc.), I recommend the GNU manuals which are very well-written: https://www.gnu.org/manual/blurbs.html (example: Bash manual);
  • GNU manuals are also downloaded for offline read by default. These manuals are provided with info.
• For GUI desktop, it depends on what Desktop Environment (DE) you use:
  • KDE wiki with tutorials: https://userbase.kde.org/Tutorials
  • GNOME app overview with documentation: https://apps.gnome.org/
  • If you don’t know what DE you use, KDE uses sharp edges for windows and GNOME uses round edges ( 👁️👄👁️ ).
• For system, as @Terumo@lemmy.world pointed out, for system-specific stuff, there’s ArchWiki that has topics about everything.
• If you want to understand what are the /usr, /usr/local, /etc, /dev, etc. filesystem directories, you want to look at the Filesystem Hierarchy Standard: https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html
• There is also a free training course for Linux: https://training.linuxfoundation.org/training/introduction-to-linux/
• If you’re a developer and want to find out how deep the rabbit hole is, you may want to look at an online book Operating Systems: Three Easy Pieces: https://pages.cs.wisc.edu/~remzi/OSTEP/

EDIT: Forgot this important material:

• If you need to know command-line argument specifics for a particular program, use manpages (For example, to find brief information about grep, type man grep in your shell, and info grep if you need a complete manual).

manpages. For many of us, it’s the only documentation that existed prior to the Internet.

Maybe tell us what you need the documentation for… Writing kernel modules? Installing distros? Configuring software? Archlinux wiki is a very good general reference, btw.

https://www.oreilly.com/library/view/linux-in-a/9780596806088/ and other O’Reilly books.

Documentation or tipps?

Tipps:

• itsfoss

Gentoo Wiki https://wiki.gentoo.org/wiki/Main_Page

Although i myself use an Arch-Based System i find the Gentoo wiki most of the time better regarding configuration of services

Surprised you didn’t think of https://docs.kernel.org/.