docs: regroup feature list using GPT and update Gave GPT 3.5 the original list of features annotated with the section header when needed to understand feature context. Asked it to recategorize. Took it's output and moved some things around, edited, and added more links. Also added links to the wiki for OAUTH and Shibboleth extrnal user databases to the customizing document section on external user databases.

# make this dir a package