Useful Resources

Arch Wiki

Useful for a overview and medium detail explanation about configurations of Linux components as well as packages. Although package names vary from distro to distro, nearly all of the information transfer.

https://wiki.archlinux.org/

Gentoo Wiki

Useful for a more technical lower level detail of Linux components.

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

Kali

https://www.kali.org/docs/
https://comparite.ch/kalics

LinuxDocs.org

http://linuxdocs.org/

Debian Docs

https://www.debian.org/doc/

Linux Foundation

https://www.linuxfoundation.org/projects/linux/

Tutorials Point

https://www.tutorialspoint.com/unix/index.htm

ibiblop.org

I collection of projects and operating systems with links to their wikis. May not be limited to Open Source, but does include them.

https://www.ibiblio.org/catalog/items/browse

die.net

Has a cache of hundreds of manpages, as well as some basics

https://linux.die.net/

The Linux Documentation Project

This is rather old, (most recent article dates from 2012), but may have some useful things

https://tldp.org/