Jenkins is an automation engine with an unparalleled plugin ecosystem to support all of your favorite tools in your delivery pipelines, whether your goal is continuous integration, automated testing, or continuous delivery.
The Jenkins Handbook is a collection of in-depth chapters describing the various important aspects of using Jenkins in detail.
The LTS upgrade guide describes possible issues and solutions for users upgrading between LTS releases.
For developers wishing to hack on, or extend Jenkins we have a number of documentation resources available. The Extend Jenkins page contains a wide variety of developer contributed documentation and can be a good start.
Plugin authors may also find the Extension Points catalogue to be a useful reference document.