The Open Web Application Security Project (OWASP) has a lot of projects focused on documentation. Some of them are general, such as the OWASP Testing Guide, which tries to describe all kinds of vulnerabilities, and how to detect, exploit, and solve them. The OWASP Development Guide summarizes the development basics for all security developers, and also documents each technology.
For a bug bounty hunter, the OWASP Testing Guide (https://www.owasp.org/index.php/OWASP_Testing_Guide_v4_Table_of_Contents) is a great reference guide, where you can go when it is necessary to solve a doubt, have a quick reference to use to create reports, or as a cheat sheet for common vulnerabilities.
