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.

# human timezones
pytz
# indexer
Whoosh
# xapian is also installed
# extra database support
psycopg2
mysqlclient
redis
# encryption
gpg
# java web tokens
PyJWT
# extra HTTP compression methods
Brotli
zstd
# jinja2 template engine
Jinja2